PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID
THE HISTORY OF THE MAMMALIAN
THE most important difference between the de-
velopment of Mammalia and Aves depends upon the
amount and distribution of the food-yolk in the ovum.
In birds, as we have seen (Ch. i.), the ovum is large and
the greater part of it so heavily charged with food-yolk
that it is unable to segment. The segmentation is con-
fined to one small portion, the germinal disc, the pro-
toplasm of which is less burdened with food-yolk than
that of the remainder of the ovum. Such partial seg-
mentation is known as meroblastic.
In Mammals, on the other hand, the ovum is small 1 ,
and contains but a slight amount of food-yolk ; the little
there is being distributed uniformly throughout. In con-
sequence of this the whole ovum is able to segment ; the
segmentation therefore belongs to the holoblastic type.
This fundamental difference in the constitution of the
ovum of Birds and Mammals is accompanied not only by
differences in the segmentation but also by impoifcant
differences, as we shall see, in the stages of development
which immediately follow segmentation. Finally, in
1 The human ovarian ovum is T ^ T to 1 | ir of an inch in diameter.
birds, as we have seen, the nutrition of the developing
embryo is entirely effected at the expense of the food-
yolk and albumen with which the ovum was charged
in the ovary and oviduct respectively, and the eggs
leave the parent very soon after the close of segmenta-
tion. In the Mammalia the absence of sufficient food-
yolk necessitates the existence of some other source of
nutriment for the embryo, and that source is mainly the
The development of Mammalia may be divided into
two periods : 1. the development within the uterus ; 2.
the development after birth.
In all the higher Mammalia the second period is very
unimportant, as compared with the first ; for the young
are born in a condition closely resembling that of the
adult of the species to which they belong. The de-
velopment during the first period takes place in the
uterus of the mother, and nutriment passes from the
maternal blood to that of the embryo by means of a
structure, to be described in detail hereafter, known as
the placenta. This difference between the development
of Birds and Mammals may be briefly expressed by saying
that the former are oviparous, while the latter are vivi-
The source of nutriment during the second period
is the Mammary glands. In certain of the lower Mam-
malia (Marsupials) the young are born in a very im-
mature condition, and become attached by their mouths
to the nipples of these glands. They are carried
about, usually in a special pouch (marsupium) by the
mother, and undergo in this position the greater part of
the remainder of their development.
GENERAL DEVELOPMENT OF THE EMBRYO.
THERE is a close agreement in the history of the
development of the embryo of the various kinds of
Mammals. We may therefore take one, the Rabbit, as
a type. There are without doubt considerable varia-
tions to be met with in the early development even of
species nearly allied to the Rabbit, but at present the
true value of these variations is not understood, and
they need not concern us here.
The ovarian ovum. Mammals possess two ovaries
situated in the body cavity, one on either side of the
vertebral column immediately posterior to the kidneys.
They are somewhat flattened irregularly oval bodies, a
portion of the surface being generally raised into pro-
tuberances due to projecting follicles.
In an early stage of development the follicle in the
mammalian ovary is similar to that of the fowl, and is
formed of flat cells derived from the germinal cells ad-
joining the ovum. As development proceeds however
it becomes remarkably modified. These flat cells sur-
rounding the ovum become columnar and then one or
two layers deep. Later they become thicker on one
side of the ovum than on the other, and there appears
310 THE MAMMALIAN EMBRYO. [CHAP.
in the thickened mass a cavity which gradually becomes
more and more distended and filled with an albuminous
As the cavity enlarges, the OYum, around which are
several layers of cells, forms a prominence projecting
into it. The follicle cells are known as the membrana
granulosa, and the projection in which the ovum lies as
the discus or cumulus proligerus. The whole structure
with its tunic is known as the Graafian follicle.
If the ovary of a mature female during the breeding
season be examined, certain of the protuberances on its
surface maybe seen to be considerably larger than others;
they are more transparent than their fellows and their
outer covering appears more tense ; these are Graafian
follicles containing nearly or quite ripe ova. Upon pierc-
ing one of these follicles with a needle-point the ovum
contained therein spirts forth together with a not incon-
siderable amount of clear fluid.
Egg Membranes. The ovum is surrounded by a
radiately striated membrane, the zona radiata, internal
to which in the nearly ripe egg a delicate membrane
has been shown, by Ed. v. Beneden, to exist. The cells
of the discus are supported upon an irregular granular
membrane external to the zona radiata. This mem-
brane is more or less distinctly separated from the zona,
and the mode of its development renders it probable
that it is the remnant of the first formed membrane
in the young ovum and is therefore the vitelline mem-
Maturation and impregnation of the ovum. As
the ovum placed in the Graafian follicle approaches
maturity the germinal vesicle assumes an excentric
X.] IMPKEGNATION. 311
position and undergoes a series of changes which have
not been fully worked out, but which probably are of
the same nature as those which have been observed in
other types (p. 17). The result of the changes is the
formation of one or more polar bodies, and the nucleus
of the mature ovum (female pronucleus).
At certain periods one or more follicles containing a
ripe ovum burst 1 , and their contents are received by
the fimbriated extremity of the Fallopian tube which
appears according to Hensen to clasp the ovary at the
time. The follicle after the exit of the ovum becomes
filled with blood and remains as a conspicuous object on
the surface of the ovary for some days. It becomes
eventually a corpus luteum. The ovum travels slowly
down the Fallopian tube. It is still invested by the
zona radiata, and in the rabbit an albuminous envelope
is formed around it in its passage downwards. Im-
pregnation takes place in the upper part of the Fallo-
pian tube, and is shortly followed by the segmentation,
which is remarkable amongst the Amniota for being
complete 2 .
The entrance of the spermatozoon into the ovum
and its subsequent fate have not been observed. Van
Beneden describes in the rabbit the formation of the
first segmentation nucleus (i.e. the nucleus of the ovum
after fertilization) from two nuclei, one peripheral and
the other ventral, and deduces from his observations
1 So far as is known there is no relation between the bursting of
the follicle and the act of coition.
2 It is stated by Bischoff that shortly after impregnation, and
before the commencement of the segmentation, the ova of the rabbit
and guinea-pig are covered with cilia and exhibit the phenomenon of
rotation. This has not been noticed by other observers.
THE MAMMALIAN EMBRYO.
that the peripheral nucleus was derived from the sper-
Segmentation. The process of segmentation oc-
cupies in the rabbit about 72 hours; but the time of
this and all other stages of development varies con-
siderably in different animals.
The details of segmentation in the rabbit are differ-
ently described by various observers ; but at the close of
segmentation the ovum appears undoubtedly to be
composed of an outer layer of cubical hyaline cells,
almost entirely surrounding an inner mass of highly
granular rounded or polygonal cells.
OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES
CLOSELY FOLLOWING UPON THE SEGMENTATION.
(After E. van Beneden.)
ep. outer layer ; %. inner mass ; bp. Van Beneden's blastopore.
The shading of the outer and inner layers is diagrammatic.
In a small circular area however the inner mass of
cells remains exposed at the surface (Fig. 95, A). This
X.] SEGMENTATION. 313
exposed spot may for convenience be called with v. Bene-
den the blastopore, though, as will be seen by the ac-
count given of the subsequent development, it in no
way corresponds with the blastopore of other vertebrate
In the following account of the segmentation of the rabbit's
ovum, v. Beneden's description is followed as far as the details
are concerned, his nomenclature is however not adhered to 1 .
According to v. Beneden the ovum first divides into two
nearly equal spheres, of which one is slightly larger and more
transparent than the other. The larger sphere and its products
will be spoken of as the outer spheres, and the smaller one
and its products as the inner spheres, in accordance with their
Both the spheres are soon divided into two, and each of the
four so formed into two again ; and thus a stage with eight
spheres ensues. At the moment of their first separation these
spheres are spherical, and arranged in two layers, one of them
formed of the four outer, and the other of the four inner spheres.
This position is not long retained, for one of the inner spheres
passes to the centre ; and the whole ovum again takes a spherical
In the next phase of segmentation each of the four outer
spheres divides into two, and the ovum thus becomes constituted
of twelve spheres, eight outer and four inner. The outer spheres
have now become markedly smaller than the inner.
The four inner spheres next divide giving rise, together with
the eight outer spheres, to sixteen spheres in all ; which are
nearly uniform in size. Of the eight inner spheres four soon
pass to the centre, while the eight now superficial outer spheres
form a kind of cup partially enclosing the inner spheres. The
outer spheres now divide in their turn, giving rise to sixteen
1 The cells spoken of as the outer layer correspond to Van Beneden's
epiblast, whilst those cells spoken of as the inner correspond to his
314 THE MAMMALIAN EMBEYO. [CHAP.
spheres which largely enclose the inner spheres. The segmenta-
tion of both outer and inner spheres continues, and in the course
of it the outer spheres spread further and further over the inner,
so that at the close of segmentation the inner spheres constitute a
central solid mass almost entirely surrounded by the outer
spheres. In a small circular area however the inner mass of
spheres remain for some time exposed at the surface (Fig. 95 A).
The blastodennic vesicle. After its segmentation
the ovum passes into the uterus. The outer cells soon
grow over the blastopore and thus form a complete
superficial layer. A series of changes next take place
which result in the formation of what has been called
the blastodermic vesicle.
These changes commence with the appearance of a
narrow cavity between the outer and inner layers, which
extends so as completely to separate them except in the
region adjoining the original site of the blastopore (Fig.
95 B) 1 . The cavity so formed rapidly enlarges, and
with it the ovum also ; so that this soon takes the form
of a thin walled vesicle with a large central cavity.
This vesicle is the blastodermic vesicle. The greater
part of its walls are formed of a single row of flattened
outer layer cells; while the inner mass of cells forms
a small lens-shaped mass attached to the inner side of
the outer layer (Fig. 96).
Although by this stage, which occurs in the rabbit
between seventy and ninety hours after impregnation,
the blastodermic vesicle has by no means attained its
greatest dimensions, it has nevertheless grown from
1 Van Beheden regards it as probable that the blastopore is
situated somewhat excentrically in relation to the area of attachment
of the inner mass to the outer layer.
about O09 mm. the size of the ovum at the close
segmentation to about 0*28 in diameter. It is en-
closed by the zona radiata and the albuminous layer
BABBIT'S OVUM BETWEEN 70 90 HOURS AFTER IMPREGNATION.
(After E. van Beneden.)
bv. cavity of blastodermic vesicle (yolk-sac) ; ep. outer layer ;
hy. inner mass ; Zp. albuminous envelope.
around it. The blastodermic vesicle continues to
enlarge rapidly, and during the process the inner mass
undergoes important changes. It spreads out on the
inner side of the outer layer and at the same time loses
its lens-like form and becomes flattened. The central
316 THE MAMMALIAN EMBRYO. [CHAP.
part of it remains however thicker, and is constituted
of two rows of cells, while the peripheral part, the outer
boundary of which is irregular, is formed of an imperfect
layer of amoeboid cells which continually spread further
and further beneath the outer layer. The central thick-
ening of the inner layer forms an opaque circular spot
on the blastoderm, which constitutes the commencement
of the embryonic area.
The formation of the layers. The history of the
stages immediately following, from about the com-
mencement of the fifth day to the seventh day, when a
primitive streak makes its appearance, is not perfectly
understood, and has been interpreted very differently by
various observers. The following account must there-
fore be considered as a tentative one.
About five days after impregnation the cells of the
inner mass in the embryonic area become divided into
two distinct strata, an upper stratum of rounded cells
adjoining the flattened outer layer and a lower stratum
of flattened cells. This lower stratum is the true hypo-
blast (Fig. 97). At the edge of the embryonic area the
hypoblast is continuous with a peripheral ring of the
amosboid cells of the earlier stage, which now form,
except at the edge of the ring, a continuous layer of
flattened cells in contact with the outer layer. During
the sixth day the middle layer becomes fused with the
outer layer, and gives rise to a layer of cells which are
columnar and are arranged in the rabbit in a single
row (Fig. 98). They form together the true epiblast of
the embryonic area.
At this stage therefore the embryonic area, which is
circular, is formed throughout of two single layers of
FORMATION OF THE LAYERS.
cells, a columnar epiblast and a layer of flattened hypo-
SECTION THROUGH THE NEARLY CIRCULAR EMBRYONIC AREA OF
A RABBIT OVUM OF Six DAYS.
(From Allen Thomson, after E. van Beneden.)
ect. upper layer ; mes. middle layer ; ent. true hypoblast.
SECTION THROUGH THE BLASTODERM OF A RABBIT ON THE
SEVENTH DAY : TAKEN IN FRONT OF THE PRIMITIVE
Half of the area is represented.
Towards the end of the sixth day the embryonic
area of the rabbit, which has hitherto been round, be-
A diagrammatic view of the whole blastodermic
vesicle at about the beginning of the seventh day is
given in Fig. 99. The embryonic area is represented in
white. The line ge in B shows the extension of the
hypoblast round the inside of the vesicle. The bias-
THE MAMMALIAN EMBRYO.
VIEWS OF THE JBLASTODERMIC VESICLE OF A KABBIT ON THE
SEVENTH DAY WITHOUT THE ZONA. A. from above, B.
from the side. (From Kolliker.)
ag. embryonic area ; ge. boundary of the hypoblast.
X.] PRIMITIVE STREAK. 319
todermic vesicle is therefore formed of three areas,
(1) the embryonic area with two layers, a columnar
epiblast and flat hypoblast; (2) the region around the
embryonic area where the walls of the vesicle are formed
of flattened epiblast 1 and of hypoblast ; (3) the area
beyond this again where the vesicle is formed of flat-
tened epiblast 1 only.
The changes which next take place begin with the
formation of a primitive streak, homologous with, and in
most respects similar to, the primitive streak in Birds.
EMBRYONIC AREA OF AN EIGHT DAYS' RABBIT.
arg. embryonic area ; pr. primitive streak.
The formation of the streak is preceded by that of a
dark spot near the middle of the blastoderm, forming
the nodal point of Hensen. This spot subsequently
constitutes the front end of the primitive streak.
Early on the seventh day the embryonic area be-
comes pyriform, and at its posterior and narrower end
1 The epiblast of the blastodermic vesicle beyond the embryonic
area is formed of the outer layer only.
320 THE MAMMALIAN EMBRYO. [CHAP.
the primitive streak makes its appearance ; it is due to
a proliferation of rounded cells from the epiblast.
SECTION THROUGH AN OVAL BLASTODERM or A RABBIT ON
THE SEVENTH DAY. THE LENGTH OF THE AREA WAS
ABOUT 1'2 MM. AND ITS BREADTH ABOUT '86 MM.
Through the front part of the primitive streak ; ep. epiblast ;
m. mesoblast ; hy. hypoblast ; pr. primitive streak.
These cells give rise to a part of the mesoblastic
layer of the embryo, and may be termed from their
origin the primitive streak mesoblast.
During the seventh day the primitive streak be-
comes a more pronounced structure (Fig. 101), the
mesoblast in its neighbourhood increases in quantity,
while an axial groove (Fig. 100) the primitive groove
is formed on its upper surface.
The formation of the medullary groove. In the
part of the embryonic area in front of the primitive
streak there arise during the eighth day two folds
bounding a shallow median groove, which meet in front,
but diverge behind, and enclose between them the
foremost end of the primitive streak (Fig. 103). These
folds are the medullary folds and they constitute the
first definite traces of the embryo. The medullary plate
bounded by them rapidly grows in length, the primitive
streak always remaining at its hinder end. While the
Two TRANSVERSE SUCTIONS THROUGH THE EMBRYONIC AREA
OF AN EMBRYO RABBIT OF SEVEN DAYS.
The embryo has nearly the appearance represented in Fig. 100.
A. is taken through the anterior part of the embryonic area.
It represents about half the breadth of the area, and there is no
trace of a medullary groove or of the mesoblast.
B. is taken through the posterior part of the primitive
ep. epiblast ; hy. hypoblast.
lateral epiblast is formed of several rows of cells, that of
the medullary plate is at first formed of but a single
row (Fig. 104, mg).
The mesoblast and notochord. The mesoblast in
mammalia has, as in the chick, a double origin, and the
details of its development appear to resemble essentially
those in the chick. It arises (1) from the epiblast of
the primitive streak ; this has been already described ;
(2) from the primitive hypoblast in front and at the
sides of the primitive streak. The latter is known as
hypoblastic mesoblast, and as in the chick appears to
originate as two lateral plates split off from the primi-
tive hypoblast. These two plates are at first continuous
F. &B. 21
THE MAMMALIAN EMBRYO. [CHAP.
EMBRYONIC AREA OF A SEVEN DAYS' EMBRYO RABBIT.
o. place of future area vasculosa ; rf. medullary groove ; pr. pri-
mitive streak ; ag. embryonic area.
In the region o. a layer of mesoblast has already grown ; there
are however as yet no signs of blood-vessels in it.
This mesoblast is derived from the mesoblast of the primitive
in the axial line with the primitive hypoblast. When
the medullary groove is formed the lateral bands of
raesoblast become separate from the axial hypoblast and
give rise to two independent lateral plates of mesoblast
X.] THE PRIMITIVE STREAK. 323
(Fig. 104). The axial band of hypoblast eventually
oives rise to the notochord.
TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT
efj. epiblast ; me. mesoblast ; Jiy. hypoblast ; mg. medullary
The mesoblastic elements from these two sources,
though at first characterised by the difference in the
appearance of their cells (Fig. 102, B), those of the
primitive streak mesoblast being more rounded, soon
become blended and indistinguishable from one another;
so that it is difficult to say to what parts of the fully
formed mesoblast they severally contribute.
In tracing the changes which take place in the rela-
tions of the layers, while passing from the region of the
embryo to that of the primitive streak, it will be con-
venient to follow the account given by Schafer for the
guinea-pig, which on this point is far fuller and more
satisfactory than that of other observers. In doing so
we shall leave out of consideration the fact that the
layers in the guinea-pig are inverted. Fig. 105 repre-
sents a series of sections through this part in the guinea-
pig. The anterior section (D) passes through the medul-
lary groove near its hinder end. The commencement of
the primitive streak is marked by a slight prominence on
the floor of the medullary groove between the two diverg-
THE MAMMALIAN EMBRYO.
ing medullary folds (Fig. 105 C, ae). Where this promi-
nence becomes first apparent the epiblast and hypoblast
A SERIES OF TRANSVERSE SECTIONS THROUGH THE JUNCTION
OF THE PRIMITIVE STREAK AND MEDULLARY GROOVE OF
A YOUNG GUINEA-PIG. (After Schafer.)
A. is the posterior section.
e. epiblast ; m. mesoblast ; h. hypoblast ; ae. axial epiblast of
the primitive streak ; ah. axial hypoblast attached in B. and
C. to the epiblast at the rudimentary blastopore ; ng. me-
dullary groove ; /. rudimentary blastopore.
X.] THE NOTOCHORD. 325
are united together. The mesoblast plates at the two
sides remain in the meantime quite free. Slightly
further back, but before the primitive groove is reached,
the epiblast and hypoblast are connected together by a
cord of cells (Fig. 105 B,/), which in the section next
following becomes detached from the hypoblast and
forms a solid keel projecting from the epiblast. In the
following section the hitherto independent mesoblast
plates become united with this keel (Fig. 105 A) ; and
in the posterior sections, through the part of the primi-
tive streak with the primitive groove, the epiblast and
mesoblast continue to be united in the axial line, but
the hypoblast remains distinct. These peculiar relations
may shortly be described by saying that in the axial
line the hypoblast becomes united with the epiblast at
the posterior end of the embryo; and that the cells
which connect the hypoblast and epiblast are posteriorly
continuous with the fused epiblast and mesoblast of
the primitive streak, the hypoblast in the region of the
primitive streak having become distinct from the other
The notochord. The thickened axial portion of the
hypoblast in the region of the embryo becomes sepa-
rated, as we have already pointed out, from the lateral
parts as the notochord.
Very shortly after the formation of the notochord,
the hypoblast grows in from the two sides, and becomes
quite continuous across the middle line. The formation
of the notochord takes place from before backwards;
and at the hinder end of the embryo it is continued
into the mass of cells which forms the axis of the primi-
tive streak, becoming therefore at this point continuous
326 THE MAMMALIAN EMBRYO. [CHAP.
with the epiblast. The notochord in fact behaves exactly
as did the axial hypoblast in the earlier stage.
The peculiar relations just mentioned are precisely similar to
those we have already described in the chick (p. 60). They
receive their explanation by comparison with the lower types.
The cells which form the junction between the epiblast and
the axial hypoblast constitute in the lower types the front wall of
a passage perforating the blastoderm and leading from the ex-
terior into the alimentary canal. This passage is the vertebrate
In the chick we have seen (p. 72) this passage is present at a
certain stage of development as the neurenteric canal ; and in the
duck at a still earlier stage. It is also present at an early stage
in the mole.
The presence of this blastopore renders it clear that the blas-
topore discovered by Ed. van Beneden cannot have the meaning
he assigned to it in comparing it with the blastopore of the
To recapitulate. At the stage we have now reached
the three layers are definitely established.
The epiblast is derived partly from the outer layer
of segmentation spheres and partly from the larger pro-
portion of those segmentation spheres which constitute
the inner mass. The hypoblast arises from the few
remaining cells of the inner mass ; while the mesoblast
has its origin partially from the epiblast of the primitive
streak and partially from the hypoblast cells anterior to
the primitive streak.
During the period in which these changes have been taking
place, the rudiments of a vascular area become formed, and while
as Kolliker has shewn, the mesoblast of this portion is to some
extent derived from the mesoblast of the primitive streak, it is
possible that a portion of it owes its origin to hypoblastic meso-
X.] THE MEDULLARY PLATE. 327
General growth of the embryo. We have seen
that the blastodermic vesicle becomes divided at an
early stage of development into an embryonic area, and
a non-embryonic portion. The embryonic area gives
rise to the whole of the body of the embryo, while the
non-embryonic part forms an appendage known as the
umbilical vesicle, which becomes gradually folded off
from the embryo, and has precisely the relations of the
yolk-sac of the chick. It is almost certain that the
Mammalia are descended from ancestors, the embryos
of which had large yolk-sacs, but that the yolk has
become reduced in quantity owing to the nutriment
received from the wall of the uterus taking the place
of that originally supplied by the yolk. A rudiment of
the yolk-sac being thus retained in the umbilical vesi-
cle, this structure may be called indifferently umbilical
vesicle or yolk-sac.
The yolk which fills the yolk-sac in Birds is re-
placed in Mammals by a coagulable fluid; while the
gradual extension of the hypoblast round the wall of
the blastodermic vesicle, which has already been de-
scribed, is of the same nature as the growth of the hy-
poblast round the yolk-sac in Birds.
The whole embryonic area would seem to be em-
ployed in the formation of the body of the embryo. Its
long axis has no very definite relation to that of the
blastodermic vesicle. The first external trace of the
embryo to appear is the medullary plate, bounded by
the medullary folds, and occupying at first the anterior
half of the -embryonic area (Fig. 103). The two me-
dullary folds diverge behind and enclose the front end
of the primitive streak. As the embryo elongates the
328 THE MAMMALIAN EMBRYO. [CHAP.
medullary folds nearly meet behind and so cut off the
front portion of the primitive streak, which then ap-
pears as a projection in the hind end of the medullary
groove. At the hind end of the medullary groove
(mole) a deep pit perforates its floor and enters the
mass of mesoblast cells lying below. The pit is a rudi-
ment of the blastopore (described on p. 326) which has
been enclosed by the medullary folds.
Henceforward the general course of development is
very similar to that in the chick and so will be only briefly
described. The special features in the development of
particular organs will be described later. In an embryo
rabbit, eight days after impregnation, the medullary
groove is about 1*80 mm. in length. At this stage a
division may be clearly seen in the lateral plates of
mesoblast into a vertebral zone adjoining the embryo
and a more peripheral lateral zone ; and in the verte-
bra] zone indications of two somites, about 0'37 mm.
from the hinder end of the embryo, become apparent.
The foremost of these somites marks the junction, or
very nearly so, of the cephalic region and trunk. The
small size of the latter as compared with the former is
very striking, but is characteristic of Vertebrates gene-
rally. The trunk gradually elongates relatively to the
head, by the addition behind of fresh somites. The
embryo has not yet begun to be folded off from the
In a slightly older embryo of nine days there appears
(Hensen, Kolliker) round the embryonic area a delicate
clear ring which is narrower in front than behind (Fig.
106 A. ap). This ring is regarded by these authors as
representing the peripheral part of the area pellucida of
X.] THE CEKEBRAL VESICLES. 329
Birds, which does not become converted into the body
of the embryo. Outside the area pellucida, an area
vasculosa has become very well defined. In the em-
bryo itself (Fig. 106 A) the disproportion between head
and trunk is less marked than before; the medullary
plate dilates anteriorly to form a spatula-shaped ce-
phalic enlargement; and three or four somites are
established. In the lateral parts of the mesoblast of
the head there may be seen on each side a tube-like
structure (hz). Each of these is part of the heart, which
arises as two independent tubes. The remains of the
primitive streak (pr) are still present behind the me-
In somewhat older embryos (Fig. 106 B) with about
eight somites, in which the trunk considerably exceeds
the head in length, the first distinct traces of the
folding off of the head end of the embryo become ap-
parent, and somewhat later a fold also appears at the
hind end. In the formation of the hind end of the
embryo the primitive streak gives rise to a tail swelling
and to part of the ventral wall of the post-anal gut. In
the region of the head the rudiments of the heart (h)
are far more definite. The medullary groove is still
open for its whole length, but in the head it exhibits a
series of well-marked dilatations. The foremost of
these (vh) is the rudiment of the fore-brain from the
sides of which there project the two optic vesicles (all) ;
the next is the mid-brain (mK) and the last is the hind--
brain (hh), which is again divided into smaller lobes by
successive constrictions. The medullary groove behind
the region of the somites dilates into an embryonic
sinus rhomboidalis like that of the bird. Traces of the
THE MAMMALIAN EMBRYO.
EMBRYO RABBITS OF ABOUT NINE DAYS FROM THE DORSAL SIDE.
A. magnified 22 times, and B. 21 times.
ap. area pellucida ; rf. medullary groove ; h f . medullary plate in
the region of the future fore-brain ; h". medullary plate in
the region of the future mid-brain ; vh. fore- brain ; ab. optic
vesicle ; mh. mid-brain ; lili. and h'". hind-brain ; uw. meso-
blastic somite ; stz. vertebral zone ; pz. lateral zone ; liz. and
h. heart ; ph. pericardial section of body-cavity ; vo. vitelline
vein ; af. amnion fold.
X.] GENERAL DEVELOPMENT. 331
amnion (of) are now apparent both in front of and
behind the embryo.
The structure of the head and the formation of the
heart at this age are illustrated in Fig. 107. The
widely open medullary groove (rf) is shewn in the
centre. Below it the hypoblast is thickened to form
the notochord dd' ; and at the sides are seen the two
tubes, which, on the folding-in of the fore-gut, give rise
to the unpaired heart 1 . Each of these is formed of
an outer muscular tube of splanchnic mesoblast (ahh),
not quite closed towards the hypoblast, and an inner
epithelioid layer (ihh), and is placed in a special section
of the body cavity (ph), which afterwards forms the
Before the ninth day is completed great external
changes are usually effected. The medullary groove
becomes closed for its whole length with the exception
of a small posterior portion. The closure commences,
as in Birds, in the region of the mid-brain. Anteriorly
the folding-off of the embryo proceeds so far that the
head becomes quite free, and a considerable portion of
the throat, ending blindly in front, becomes established.
In the course of this folding the, at first widely sepa-
rated, halves of the heart are brought together, coalesce
on the ventral side of the throat, and so give rise to a
median undivided heart. The fold at the tail end of
the embryo progresses considerably, and during its ad-
vance the allantois is formed in the same way as in
Birds. The somites increase in number to about twelve.
The amniotic folds nearly meet above the embryo.
1 The details of the development of the heart are described below
THE MAMMALIAN EMBRYO, [CHAP.
TRANSVERSE SECTION THROUGH THE HEAD OF A KABBIT OF
THE SAME AGE AS FIG. 106 B. (From Kolliker.)
B. is a more highly magnified representation of part of A.
rf. medullary groove ; mp. medullary plate ; rw. medullary fold ;
h. epiblast ; dd. hypoblast ; dd'. notochordal thickening of
hypoblast ; sp. undivided mesoblast ; hp. somatic mesoblast ;
X.] THE CRANIAL FLEXURE. 333
dfjj. splanchnic mesoblast; ph. pericardial section of body-
cavity ; ahh. muscular wall of heart ; ihh. epithelioid layer of
heart ; mes. lateral undivided mesoblast ; sw. fold of hypo-
blast which will form the ventral wall of the pharynx ; sr.
The later stages in the development proceed in the
main in the same manner as in the Bird. The cranial
flexure soon becomes very marked, the mid-brain form-
ing the end of the long axis of the embryo (Fig. 108).
The sense organs have the usual development. Under
the fore -brain appears an epiblastic involution giving
ADVANCED EMBRYO OF A RABBIT (ABOUT TWELVE DAYS) 1 .
mb. mid-brain ; ih. thalamencephalon ; ce. cerebral hemisphere ;
op. eye ; iv.v. fourth ventricle ; moc. maxillary process ; md.
mandibular arch ; Jiy. hyoid arch ; fl. fore-limb ; hi. hind-
limb ; urn. umbilical stalk.
1 .This figure was drawn by Mr Weldon.
334 THE MAMMALIAN EMBRYO. [CHAP.
rise both to the mouth and to the pituitary body. Be-
hind the mouth are three well marked pairs of visceral
arches. The first of these is the mandibular arch
(Fig. 108 md) y which meets its fellow in the middle
line, and forms the posterior boundary of the mouth.
It sends forward on each side a superior maxillary pro-
cess (mx) which partially forms the anterior margin of
the mouth. Behind the mandibular arch are present a
well-developed hyoid (hy) and a first branchial arch
(not shewn in Fig. 108). There are four clefts, as in
the chick, but the fourth is not bounded behind by a
definite arch. Only the first of these clefts persists as
the tympanic cavity and Eustachian tube.
At the time when the cranial flexure appears, the
body also develops a sharp flexure immediately behind
the head, which is thus bent forwards upon the pos-
terior straight part of the body (Fig. 108). The amount
of this flexure varies somewhat in different forms. It
is very marked in the dog (Bischoff ). At a later period,
and in some species even before the stage figured, the
tail end of the body also becomes bent (Fig. 108), so
that the whole dorsal side assumes a convex curvature,
and the head and tail become closely approximated. In
most cases the embryo, on the development of the tail,
assumes a more or less definite spiral curvature (Fig.
108). With the more complete development of the
lower wall of the body the ventral flexure partially dis-
appears, but remains more or less persistent till near
the close of intra-uterine life. The limbs are formed as
simple buds in the same manner as in Birds. The buds
of the hind-limbs are directed somewhat forwards, and
those of the fore-limb backwards.
X.] THE HUMAN EMBRYO. 335
The human embryo. Our knowledge as to the
early development of the human embryo is in an un-
satisfactory state. The positive facts we know are com-
paratively few, and it is not possible to construct from
them a history of the development which is capable of
satisfactory comparison with that in other forms, unless
all the early embryos known are to be regarded as
abnormal. The most remarkable feature in the develop-
ment, which was first clearly brought to light by Allen
Thomson in 1839, is the very early appearance of
branched villi. In the last few years several ova, even
younger than those described by Allen Thomson, have
been met with, which exhibit this peculiarity.
The best preserved of these ova is one described by
Reichert 1 . This ovum, though probably not more than
thirteen days old, was completely enclosed by a decidua
reflexa. It had (Fig. 109 A and B) a flattened oval
form, measuring in its two diameters 5 '5 mm. and
3*5 mm. The edge was covered with branched villi,
while in the centre of each of the flattened surfaces
there was a spot free from villi. On the surface ad-
joining the uterine wall was a darker area (e) formed of
two layers of cells. Nothing certain has been made out
about the structure of ova of this age.
The villi, which at first leave the flattened poles
free, seem soon to extend first over one of the flat sides
and finally over the whole ovum (Fig. 109 C).
Unless the two-layered region of Reichert's ovum is
the embryonic area, nothing which can clearly be
identified as an embryo has been detected in these
1 Abhandlungen der Konigl. Akad. d. Wiss. zu Berlin, 1873.
THE MAMMALIAN EMBRYO.
THE HUMAN OVA DURING EARLY STAGES OP DEVELOPMENT.
(From Quain's Anatomy.)
A. and B. Front and side view of an ovum figured by Keichert,
supposed to be about thirteen days. e. embryonic area.
C. An ovum of about four or five weeks shewing the general
structure of the ovum before the formation of the placenta.
Part of the wall of the ovum is removed to shew the embryo
in situ. (After Allen Thomson.)
early ova. In an ovum described by Breus, and in one
described long ago by Wharton-Jones, a mass found in
the interior of the ovum may perhaps be interpreted
(His) as the remains of the yolk. It is, however, very
probable that all the early ova so far obtained are
more or less pathological.
The youngest ovum with a distinct embryo is one
described by His. This ovum, which is diagrammati-
cally represented in Fig. Ill in longitudinal section,
had the form of an oval vesicle completely covered by
villi, being about 8*5 mm. and 5*5 mm. in its two
diameters, and flatter on one side than on the other.
An embryo with a yolk-sac was attached to the inner
side of the flatter wall of the vesicle by a stalk, which
must be regarded as the allantoic stalk; the embryo
THE HUMAN EMBRYO.
THREE EARLY HUMAN EMBRYOS. (Copied from His.)
A. Side view of an early embryo described by His.
B. Embryo of about 12 14 days described by Allen Thom-
C. Young embryo described by His.
am. amnion ; md. medullary groove ; um. umbilical vesicle ;
ck. chorion, to which the embryo is attached by a stalk.
and yolk-sac filled up but a very small part of the
whole cavity of the vesicle.
The embryo, which was probably not quite normal
(Fig. 110 A), was very imperfectly developed; a me-
dullary plate was hardly indicated, and, though the
mesoblast was unsegmented, the head fold, separating
the embryo from the yolk-sac (um), was already in-
F. & B. 22
338 THE MAMMALIAN EMBRYO. [CHAP.
DIAGRAMMATIC LONGITUDINAL SECTION OF THE OVUM TO
WHICH THE EMBRYO (FiG. 110 A.) BELONGED. (After His.)
am. amnion ; Nb. umbilical vesicle.
dicated. The amnion (am) was completely formed, and
vitelline vessels had made their appearance.
Two embryos described by Allen Thomson are but
slightly older than the above embryo of His. Both of
them probably belong to the first fortnight of preg-
nancy. In both cases the embryo was more or less
folded off from the yolk-sac, and in one of them the
medullary groove was still widely open, except in the
region of the neck (Fig. 110 B). The allantoic stalk, if
present, was not clearly made out, and the condition of
the amnion was also not fully studied. The smaller of
the two ova was just 6 mm. in its largest diameter, and
was nearly completely covered with simple villi, more
developed on one side than on the other.
In a somewhat later period, about the stage of a
chick at the end of the second day, the medullary folds
are completely closed, the region of the brain already
marked, and the cranial flexure commencing. The
mesoblast is divided up into numerous somites, and the
mandibular and first two branchial arches are indicated.
THE HUMAN EMBRYO.
The embryo is still but incompletely folded off from
the yolk-sac below.
In a still older stage the cranial flexure becomes
still more pronounced, placing the mid-brain at the end
of the long axis of the body. The body also begins to
be ventrally curved (Fig. 110 C).
Externally human embryos at this age are charac-
terized by the small size of the anterior end of the
The flexure goes on gradually increasing, and in the
third week of pregnancy in embryos of about 4 mm. the
limbs make their appearance.
The embryo at this stage (Fig. 112), which is about
Two VIEWS OP A HUMAN EMBRYO OF BETWEEN THE THIRD
AND FOURTH WEEK.
A. Side view. (From Kolliker ; after Allen Thomson.) a.
amnion ; b. umbilical vesicle ; c. mandibular arch ; e. hyoid
arch; /. commencing anterior limb; g. primitive auditory
vesicle ; h. eye ; i. heart.
B. Dorsal view to shew the attachment of the dilated allantoic
stalk to the chorion. (From a sketch by Allen Thomson.)
am. amnion ; all. allantois ; ys. yolk-sac.
THE MAMMALIAN EMBRYO.
equivalent to that of a chick on the fourth day, re-
sembles in almost every respect the normal embryos of
the Amniota. The cranial flexure is as pronounced as
usual, and the cerebral region has now fully the normal
size. The whole body soon becomes flexed ventrally,
and also somewhat spirally. The yolk-sac (B ; ys) forms a
small spherical appendage with a long wide stalk, and
the embryo is attached by an allantoic stalk with a
slight swelling, probably indicating the presence of a
small hypoblastic diverticulum, to the inner face of the
A detailed history of the further development of
the human embryo does not fall within the province of
FIGURES SHEWING THE EARLY CHANGES IN THE FORM OF THE
HUMAN HEAD. (From QM&IU'S Anatomy.)
A. Head of an embryo of about four weeks. (After
B. Head of an embryo of about six weeks. (After Ecker.)
C. Head of an embryo of about nine weeks.
1. mandibular arch ; 1'. persistent part of hyomandibular cleft ;
a. auditory vesicle.
INVERSION OF THE LAYERS.
this work; while the later changes in the embryonic
membranes will be dealt with in the next chapter. For
the changes which take place on the formation of the
face we may refer the reader to Fig. 113. For a full dis-
cussion as to the relation between the human embryos
just described and those of other Mammals, we refer the
reader to the Comp. Embryology, Vol. II. p. 224 et seq.
The guinea pig, rat and mouse present a pe-
culiar method of development, the details of which are
not entirely understood, and we do not propose to
examine them here. Suffice it to say that the mode of
development gives rise to the so-called inversion of the
layers; so called because the outer layer of the em-
bryonic vesicle appeared to the older observers to be
formed of hypoblast and the embryonic epiblast to be
EMBRYONIC MEMBRANES AND YOLK-SAC.
IN the Mammalia the early stages in the develop-
ment of the embryonic membranes are nearly the same
as in Aves ; but during the later stages the allantois
enters into peculiar relations with the uterine walls,
and the two, together with the interposed portion of
the sub zonal membrane or false amnion (the nature of
which will be presently described), give rise to a very
characteristic Mammalian organ the placenta into
the structure of which it will be necessary to enter
at some length. The embryonic membranes vary so
considerably in the different forms that it will be ad-
vantageous to commence with a description of their
development in an ideal case.
We may commence with a blastodermic vesicle closely
invested by the delicate remnant of the zona radiata at
the stage in which the medullary groove is already
established. Around the embryonic area a layer of
mesoblast would have extended for a certain distance ;
so as to give rise to an area vasculosa, in which how-
ever the blood-vessels would not have become definitely
CHAP. XI.] MEMBRANES OF RABBIT. 343
established. Such a vesicle is represented diagram-
matically in Fig. 114, I. Somewhat later the embryo
begins to be folded off first in front and then behind
(Fig. 114, 2). These folds result in a constriction sepa-
rating the embryo and the yolk-sac (ds), or as it is
called in Mammalian embryology, the umbilical vesicle.
The splitting of the mesoblast into a splanchnic and a
somatic layer has taken place, and at the front and
hind end of the embryo a fold (ks) of the somatic meso-
blast and epiblast begins to rise up and grow over the
head and tail of the embryo. These two folds form the
commencement of the amnion. The head and tail folds
of the amnion are continued round the two sides of the
embryo till they meet and unite into a continuous fold.
This fold grows gradually upwards, but before it has
completely enveloped the embryo the blood-vessels of
the area vasculosa become fully developed. They are
arranged in a manner not very different from that in
The following is a brief account of their arrange-
ment in the rabbit :
The outer boundary of the area, which is continually extend-
ing further and further round the umbilical vesicle, is marked by
a venous sinus terminalis (Fig. 114, st). The area is not, as in
the chick, a nearly complete circle, but is in front divided by a
deep indentation extending inwards to the level of the heart. In
consequence of this indentation the sinus terminalis ends in
front in two branches, which bend inwards and fall directly into
the main vitelliue veins. The blood is brought from the dorsal
aortse by a series of lateral vitelline arteries, and not by a single
pair as in the chick. These arteries break up into a more deeply
situated arterial network, from which the blood is continued
partly into the sinus terminalis, and partly into a superficial venous
344 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
XI.] EMBRYONIC MEMBRANES. 345
FIVE DIAGRAMMATIC FIGURES ILLUSTRATING THE FORMATION
OF THE FOETAL MEMBRANES OF A MAMMAL. (From Kolli-
In 1, 2, 3, 4 the embryo is represented in longitudinal section.
1. Ovurn with zona pellucicla, blastodermic vesicle, and
2. Ovum with commencing formation of umbilical vesicle
3. Ovum with amnion about to close, and commencing
4. Ovum with villous subzonal membrane, larger allantois,
and mouth and anus.
5. Ovum in which the mesoblast of the allantois has ex-
tended round the inner surface of the subzonal membrane and
united with it to form the chorion. The cavity of the allantois
is aborted. This fig. is a diagram of an early human ovum.
d. zona radiata ; d and sz. processes of zona ; sh. subzonal mem-
brane, outer fold of amnion, false amnion ; ch. chorion ; ch. z.
chorionic villi ; am. amnion ; ks. head-fold of amnion ; ss. tail-
fold of amnion ; a. epiblast of embryo ; a. epiblast of non-em-
bryonic part of the blastodermic vesicle ; m. embryonic meso-
blast ; m'. non-embryonic mesoblast ; df. area vasculosa ; st.
sinus terminalis; dd. embryonic hypoblast; i. non-embryo-
nic hypoblast ; kh. cavity of blastodermic vesicle, the greater
part of which becomes the cavity of umbilical vesicle ds. ;
dg. stalk of umbilical vesicle ; al. allantois ; e. embryo ; r.
space between chorion and amnion containing albuminous
fluid ; vl. ventral body wall ; hh. pericardial cavity.
346 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
network. The hinder end of the heart is continued into two
vitelline veins, each of which divides into an anterior and a
posterior branch. The anterior branch is a limb of the sinus
terminalis, and the posterior and smaller branch is continued
towards the hind part of the sinus, near which it ends. On its
way it receives, on its outer side, numerous branches from the
venous network. The venous network connects by its anasto-
moses, the posterior branch of the vitelline vein and the sinus
Shortly after the establishment of the circulation of
the yolk-sac the folds of the amnion meet and coalesce
above the embryo (Fig. 114, 3 and 4, am). After this the
inner or true amnion becomes severed from the outer
or false amnion, though the two sometimes remain con-
nected by a narrow stalk. The space between the true
and false amnion is a continuation of the body cavity.
The true amnion consists of a layer of epiblastic epi-
thelium and generally also of somatic mesoblast, while
the false amnion consists as a rule of epiblast only;
though it is possible that in some cases (the rabbit ?)
the mesoblast may be continued along its inner
Before the two limbs of the amnion are completely
severed the epiblast of the umbilical vesicle becomes sepa-
rated from the subjacent mesoblast and hypoblast of the
vesicle (Fig. 114, 3), and, together with the false am-
nion (sh) with which it is continuous, forms a complete
lining for the inner face of the zona radiata. The space
between this membrane and the umbilical vesicle with
the attached embryo is obviously continuous with the
body cavity (vide Figs. 114, 4 and 115). To this mem-
brane Turner has given the appropriate name of sub-
zonal membrane : by Von Baer it was called the serous
XI.] ATTACHMENT OF THE OVUM. 347
envelope. It soon fuses with the zona radiata, or at
any rate the zona ceases to be distinguishable.
While the above changes have been taking place
the whole blastodermic vesicle, still enclosed in the
zona, has become attached to the walls of the uterus.
In the case of the typical uterus with two tubular
horns, the position of each embryo, when there are
several, is marked by a swelling in the walls of the
uterus, preparatory to the changes in the wall which
take place on the formation of the placenta. In the
region of each swelling the zona around the blasto-
dermic vesicle is closely embraced in a ring-like fashion
by the epithelium of the uterine wall. The whole
vesicle assumes an oval form, and it lies in the uterus
with its two ends free. The embryonic area is placed
close to the mesometric attachment of the uterus. In
many cases peculiar processes or villi grow out from
the ovum (Fig. 114, 4, sz) which fit into the folds of
the uterine epithelium, The nature of these processes
requires further elucidation, but in some instances
they appear to proceed from the zona (rabbit) and in
other instances from the subzonal membrane (dog).
In any case the attachment between the blastodermic
vesicle and the uterine wall becomes so close at the
time when the body of the embryo is first formed out
of the embryonic area, that it is hardly possible to
separate them without laceration ; and at this period
from the 8th to the 9th day in the rabbit it requires
the greatest care to remove the ovum from the uterus
without injury. It will be understood of course that
the attachment above described is at first purely super-
ficial and not vascular.
348 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
During the changes above described as taking place
in the amnion, the allantois grows out from the hind-
gut as a vesicle lined by hypoblast, but covered ex-
ternally by a layer of splanchnic mesoblast (Fig. 114, 3
and 4, at) 1 . It soon becomes a flat sac, projecting into
the now largely developed space between the subzonal
membrane and the amnion, on the dorsal side of the
embryo (Fig. 115, ALC}. In some cases it extends so
as to cover the whole inner surface of the subzonal
membrane ; in other cases again its extension is much
more limited. Its lumen may be retained or may be-
come nearly or wholly aborted. A fusion takes place
between the subzonal membrane and the adjoining
mesoblastic wall of the allantois, and the two together
give rise to a secondary membrane round the ovum
known as the chorion. Since however the allantois
does not always come in contact with the whole inner
surface of the subzonal membrane the term chorion is
apt to be somewhat vague ; in the rabbit, for instance,
a considerable part of the so-called chorion is formed
by a fusion of the wall of the yolk-sac with the sub-
zonal membrane (Fig. 116). The region of the chorion
which gives rise to the placenta may in such cases be
distinguished as the true chorion from the remaining
part which will be called the false chorion.
The mesoblast of the allantois, especially that part
of it which assists in forming the chorion, becomes
highly vascular ; the blood being brought to it by two
allantoic arteries continued from the terminal bifur-
1 The hypoblastic element in the allantois is sometimes very much
reduced, so that the allantois maybe mainly formed of a vascular layer
DIAGRAM OF THE FCETAL MEMBRANES OP A MAMMAL. (From
Structures which either are or have been at an earlier period
of development continuous with each other are represented by
the same character of shading.
pc. zona with villi ; sz. subzonal membrane ; E. epiblast of
embryo ; am. amnion ; AC. amniotic cavity ; M. mesoblast
of embryo ; H. hypoblast of embryo ; UV. umbilical vesicle ;
al. allantois ; ALC. allantoic cavity.
cation of the dorsal aorta, and returned to the body
by one, or rarely two, allantoic veins, which join the
vitelline veins from the yolk-sac. From the outer sur-
face of the true chorion (Fig. 114, 5, ch. z, 116) villi grow
out and fit into crypts or depressions which have in the
350 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
meantime made their appearance in the walls of the
uterus 1 . The villi of the chorion are covered by an
epithelium derived from the subzonal membrane, and
are provided with a connective-tissue core containing
an artery and vein and a capillary plexus connecting
them. In most cases they assume a more or less ar-
borescent form, and have a distribution on the surface
of the chorion varying characteristically in different
species. The walls of the crypts into which the villi
are fitted also become highly vascular, and a nutritive
fluid passes from the maternal vessels of the placenta
to the foetal vessels by a process of diffusion; while
there is probably also a secretion by the epithelial
lining of the walls of the crypts, which becomes ab-
sorbed by the vessels of the fcetal villi. The above
maternal and foetal structures constitute together the
organ known as the placenta. The maternal portion
consists essentially of the vascular crypts in the
uterine walls, and the foetal portion of more or less
arborescent villi of the true chorion fitting into these
While the placenta is being developed the folding
off of the embryo from the yolk-sac becomes more
complete; and the yolk-sac remains connected with the
ileal region of the intestine by a narrow stalk, the vi-
telline duct (Fig. 114, 4 and 5 and Fig. 115), consisting
of the same tissues as the yolk-sac, viz. hypoblast and
splanchnic mesoblast. While the true splanchnic stalk
1 These crypts have no connection with the openings of glands in
the walls of the uterus. They are believed by Ercolani to be formed
to a large extent by a regeneration of the lining tissue of the uterine
XL] THE PLACENTA. S51
of the yolk-sac is becoming narrow, a somatic stalk
connecting the amnion with the walls of the embryo is
also formed, and closely envelopes the stalk both of the
allantois and the yolk-sac. The somatic stalk together
with its contents is known as the umbilical cord. The
mesoblast of the somatopleuric layer of the cord de-
velops into a kind of gelatinous tissue which cements
together the whole of the contents. The allantoic ar-
teries in the cord wind in a spiral manner round the
allantoic vein. The yolk-sac in many cases atrophies
completely before the close of intra-uterine life, but in
other cases it, like the other embryonic membranes, is
not removed till birth. The intra-embryonic portion of
the allantoic stalk gives rise to two structures, viz. to
(1) the urinary bladder formed by a dilatation of its
proximal extremity, and to (2) a cord known as the
urachus connecting the bladder with the wall of the
body at the umbilicus. The urachus, in cases where
the cavity of the allantois persists till birth, remains as
an open passage connecting the intra- and extra-em-
bryonic parts of the allantois. In other cases it gradually
closes, and becomes nearly solid before birth, though a
delicate but interrupted lumen would appear to persist
in it. It eventually gives rise to the ligamentum vesicae
At birth the foetal membranes, including the foetal
portion of the placenta, are shed ; but in many forms
the interlocking of the foetal villi with the uterine
crypts is so close that the uterine mucous membrane is
carried away with the foetal part of the placenta. It
thus comes about that in some placentae the maternal
and foetal parts simply separate from each other at birth,
352 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
and that in others the two remain intimately locked
together, and both are shed together as the after-birth.
These two forms of placenta are distinguished as non-
deciduate and deciduate, but no sharp line can be drawn
between the two types. Moreover, a larger part of the
uterine mucous membrane than that actually entering
into the maternal part of the placenta is often shed in
the deciduate Mammalia, and in the non-deciduate
Mammalia it is probable that the mucous membrane
(not including vascular parts) of the maternal placenta
is either shed or absorbed.
Comparative history of the Mammalian foetal
Two groups of Mammalia the Monotremata and
the Marsupialia are believed not to be provided with
a true placenta. Nothing is known of the arrangement
of the foetal membranes in the former group of animals
(Monotremata). In the latter (Marsupialia) the yolk-
sac is large and vascular, and is, according to Owen,
attached to the subzonal membrane. The allantois on
the other hand is but small, and is not attached to the
subzonal membrane; it possesses however a vascular
Observations have hitherto been very limited with
regard to the foetal membranes of this group of animals,
but it appears highly probable that both the yolk-sac
and the allantois receive nutriment from the walls of
All Mammalia other than the Monotremata and
Marsupialia have a true allantoic placenta. The pla-
XI.] DISCOIDAL PLACENTA. 353
centa presents a great variety of forms, and we propose
first to treat the most important of these in succession,
and then to give a general exposition of their mutual
The discoidal placenta is found in the Rodentia,
Insectivora, and Cheiroptera. The Rabbit may be
taken as an example of this type of placenta.
The Rabbit. In the pregnant female Rabbit several ova are
generally found in each horn of the uterus. The general condi-
tion of the foetal-membranes at the time of their full development
is shewn in Fig. 116.
The embryo is surrounded by the amnion, which is compara-
tively small. The yolk-sac (ds) is large and attached to the
embryo by a long stalk. It has the form of a flattened sac
closely applied to about two-thirds of the surface of the subzonal
membrane. The outer wall of this sac, adjoining the subzonal
membrane, is formed of hypoblast only ; but the inner wall is
covered by the mesoblast of the area vaaculosa, as indicated by
the thick black line (fd). The vascular area is bordered by
the sinus terminalis (st). In an earlier stage of development the
yolk-sac had not the compressed form represented in the figure.
It is, however, remarkable that the vascular area never extends
over the whole yolk-sac ; but the inner vascular wall of the yolk-
sac fuses with the outer wall, and with the subzonal membrane,
and so forms a false chorion, which receives its blood supply
from the yolk-sac. This part of the chorion does not develop
The allantois (al) is a simple vascular sac with a large cavity.
Part of its wall is applied to the subzonal membrane, and gives rise
to the true chorion from which there project numerous vascular
villi. These fit into corresponding uterine crypts. It seems pro-
bable, from BischofFs and Kolliker's observations, that the sub-
zonal membrane in the area of the placenta becomes attached,
by means of villi, to the uterine wall even before its fusion with
the allantois. In the later periods of gestation the intermingling
of the maternal and fcetal parts of the placenta becomes very
F. & B. 23
354 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
close, and the placenta is truly deciduate. The cavity of the
allantois persists till birth. Between the yolk-sac, the allantois,
and the embryo, there is left a large cavity filled with an albumi-
DIAGRAMMATIC LONGITUDINAL SECTION OF A RABBIT'S OVUM
AT AN ADVANCED STAGE OF PREGNANCY. (From Kolliker
e. embryo ; a. amnion ; a. urachus ; al. allantois with blood-
vessels ; sh. sub-zonal membrane ; pi. placental villi ; fd.
vascular layer of yolk-sac; ed. hypoblastic layer of yolk-
sac ; ed'. inner portion of hypoblast, and ed". outer portion
of hypoblast lining the compressed cavity of the yolk-sac ;
ds. cavity of yolk-sac ; st. sinus terminalis ; r. space filled
with fluid between the amnion, the allantois and the yolk-
The metadiscoidal type of placenta is found in
Man and the Apes. The placenta of Man may be con-
veniently taken as an example of this type.
XI.] METADISCOIDAL PLACENTA. 355
Man. The early stages in the development of the foetal
membranes in the human embryo have not been satisfactorily
observed ; but it is known that the ovum, shortly after its
entrance into the uterus, becomes attached to the uterine wall,
which in the meantime has undergone considerable preparatory
changes. A fold of the uterine wall appears to grow round the
blastodermic vesicle, and to form a complete capsule for it, but
the exact mode of formation of this capsule is a matter of infer-
ence and not of observation. During the first fortnight of preg-
nancy villi grow out, over the whole surface of the ovum. The
further history of the early stages is extremely obscure : what
is known with reference to it will be found on p. 335 et seq. ; we
will here take up the history at about the fourth week.
At this stage a complete chorion has become formed, and is
probably derived from a growth of the niesoblast of the allantois
(unaccompanied by the hypoblast) round the whole inner surface
of the subzonal membrane. From the whole surface of the
chorion there project branched vascular processes, covered by
an epithelium. The allantois is without a cavity, but a hypo-
blastic epithelium is present in the allantoic stalk, though
not forming a continuous tube. The blood-vessels of the
chorion are derived from the usual allantoic arteries and vein.
The general condition of the embryo and of its membranes at
this period is shewn diagrammatically in Fig. 114, 5. Around
the embryo is seen the amnion, already separated by a consider-
able interval from the embryo. The yolk-sac is shewn at ds.
Eelatively to the other parts it is considerably smaller than
it was at an earlier stage. The allantoic stalk is shewn at al.
Both it and the stalk of the yolk-sac are enveloped by the
amnion, am. The chorion with its vascular processes surrounds
the whole embryo.
It may be noted that the condition of the chorion at this
stage is very similar to that of the normal diffused type of pla-
centa, described in the sequel.
While the above changes are taking place in the embryonic
membranes, the blastodermic vesicle greatly increases in size, and
forms a considerable projection from the upper wall of the
uterus. Three regions of the uterine wall, in relation to the
356 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
blastodermic vesicle, are usually distinguished; and since the
superficial parts of all of these are thrown off with the after- birth,
each of them is called a decidua. They are represented at a
somewhat later stage in Fig. 117. There is (1) the part of the
wall reflected over the blastodermic vesicle, called the decidua
reflexa (dr) ; (2) the part of the wall forming the area round
which the reflexa is inserted, called the decidua serotina (ds) ; (3)
the general wall of the uterus, not related to the embryo, called
the decidua vera (du).
The decidua reflexa and serotina together envelop the chorion
(Fig. 114. 5), the processes of which fit. into crypts in them.
At this period both of them are highly and nearly uniformly
vascular. The general cavity of the uterus is to a large extent
obliterated by the ovum, but still persists as a space filled with
mucus, between the decidua reflexa and the decidua vera.
The changes which ensue from this period onwards are fully
known. The amnion continues to dilate (its cavity being tensely
filled with amniotic fluid) till it comes very close to the chorion
(Fig. 117, am); from which, however, it remains separated by a
layer of gelatinous tissue. The villi of the chorion in the region
covered by the decidua reflexa, gradually cease to be vascular,
and partially atrophy, but in the region in contact with the
decidua serotina increase and become more vascular and more
arborescent (Fig. 117, z). The former region becomes known as
the chorion Iceve, and the latter as the chorion frondosum. The
chorion frondosum, together with the decidua serotina, gives rise
to the placenta.
The umbilical vesicle (Fig. 117, rib\ although it becomes
greatly reduced in size and flattened, persists in a recognisable
form till the time of birth.
The decidua reflexa, by the disappearance of the vessels in the
chorion Iseve, becomes non-vascular. Its tissue and that of the
decidua vera undergo changes which we do not propose to
describe here ; it ultimately fuses on the one hand with the
chorion, and on the other with the decidua vera. The mem-
brane resulting from its fusion with the latter structure becomes
thinner and thinner as pregnancy advances, and is reduced to a
thin layer at the time of birth.
DIAGRAMMATIC SECTION OF PREGNANT HUMAN UTERUS WITH
CONTAINED F<ETU8. (From Huxley after Longet.)
<il. allantoic stalk ; nb. umbilical vesicle ; am. amnion ; ch. cho-
rion ; <&. decidua serotina ; du. decidua vera ; dr. decidua
reflexa ; I. fallopian tube ; c. cervix uteri ; u. uterus ; z. foetal
villi of true placenta; ^. villi of non-placental part of
The placenta has a somewhat discoidal form, with a slightly
convex uterine surface and a concave embryonic surface. At its
edge it is continuous both with the decidua reflexa and decidua
vera. Near the centre of the embryonic surface is implanted the
umbilical cord. As has already been mentioned, the placenta is
formed of the decidua serotina and the foetal villi of the chorion
frondosum. The fcetal and maternal tissues are far more closely
united than in the placenta of the rabbit. The villi of the
chorion, which were originally comparatively simple, become
more and more complicated, and assume an extremely arborescent
form. At birth the whole placenta, together with the fused de-
358 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
cidua vera, and reflexa, with which it is continuous, is shed ; and
the blood-vessels thus ruptured are closed by the contraction of
the uterine walls.
The metadiscoidal placenta of Man and Apes and the discoidal
placenta of the Eabbit are usually classified by anatomists as
discoidal placentae, but it must be borne in mind that they differ
In the Eabbit there is a dorsal placenta, which is co- extensive
with the area of contact between the allantois and the subzonal
membrane, while the yolk-sac adheres to a large part of the
subzonal membrane. In Apes and Man the allantois spreads
over the whole inner surface of the subzonal membrane ; the
placenta is on the ventral side of the embryo, and occupies only a
small part of the surface of the allantois.
Zonary placenta. Another form of deciduate pla-
centa is known as the zonary. This form of placenta
occupies a broad zone of the chorion, leaving the two
poles free. It is found in the Carnivora, Hyrax, Elephas,
In the Dog, which may be taken as a type, there is a large
vascular yolk-sac formed in the usual way, which does not how-
ever fuse with the chorion. It has at first an oval shape, and
persists till birth. The allantois first grows out on the dorsal
side of the embryo, where it coalesces with the subzonal mem-
brane, over a small discoidal area, and there is thus formed a
rudimentary discoidal placenta closely resembling that of the
The area of adhesion between the outer part of the allantois
and subzonal membrane gradually spreads over the whole inte-
rior of the subzonal membrane, and vascular villi are formed over
the whole area of adhesion except at the two extreme poles of the
With the full growth of the allantois there is formed a broad
placental zone, with numerous branched villi fitting into corre-
sponding pita which are not true glands but special develop-
XT.] NON-DECIDUATE PLACENTA. 359
ments of the uterine surface. The maternal and foetal structures
become closely interlocked and highly vascular ; and at birth a
large part of the maternal part is carried away with the placenta ;
some of it however still remains attached to the muscular wall of
the uterus. The zone of the placenta diminishes greatly in pro-
portion to the chorion as the latter elongates, and at the full
time the breadth of the zone is not more than about one-fifth of
the whole length of the chorion.
At the edge of the placental zone there is a very small portion
of the uterine mucous membrane reflected over the non-placental
part of the chorion, so as to form a small reflexa analogous with
the reflexa in Man.
The most important of the remaining types of pla-
centa are the diffuse and the polycotyledonary, and
these placente are for the most part non-deciduate. In
the diffuse placenta, found in the Horse, Pig, Le-
murs, etc., the allantois completely envelopes the em-
bryo, and villi are formed on all parts of the chorion,
excepting over a small area at the two poles.
In the polycotyledonary placenta, which is charac-
teristic of the Ruminantia, the allantois grows round the
whole inner surface of the subzonal membrane ; the
placental villi are however not uniformly distributed,
but collected into patches or cotyledons, which form as
it were so many small placentae. The foetal villi of
these patches fit into corresponding pits in thickened
patches of the wall of the uterus.
Comparative histology of the Placenta.
It does not fall within the province of this work to
treat from a histological standpoint the changes which
take place in the uterine walls during pregnancy. It
will, however, be convenient to place before the reader
360 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
a short statement of the relations between the maternal
and foetal tissues in the different varieties of placenta.
The simplest known condition of the placenta is
that found in the pig (Fig. 118 II.). The papilla-like
foetal villi fit into the maternal crypts. The villi (v) are
formed of a connective tissue core with capillaries, and
are covered by a layer of very flat epithelium (e) de-
rived from the subzonal membrane. The maternal
crypts are lined by the uterine epithelium (e), imme-
diately below which is a capillary plexus. The maternal
and fcetal vessels are here separated by a double epi-
thelial layer. The same general arrangement holds
good in the diffused placentae of other forms, and in the
polycotyledonary placenta of the Ruminantia, but the
foetal villi in the latter (III.) acquire an arborescent form.
The maternal vessels retain the form of capillaries.
In the deciduate placenta a much more compli-
cated arrangement is usually found. In the typical
zonary placenta of the fox and cat (IV. and V.), the
maternal tissue is broken up into a complete trabecuiar
meshwork, and in the interior of the trabeculse there
run dilated maternal capillaries (d'). The trabeculse
are covered by a more or less columnar uterine epi-
thelium (e), and are in contact on every side with foetal
villi. The capillaries of the foetal villi preserve their
normal size, and the villi are covered by a flat epithelial
In the Sloth (VI.) which has a discoidal placenta the
maternal capillaries become still more dilated, and the
epithelium covering them is formed of very flat poly-
HISTOLOGY OF THE PLACENTA.
362 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP.
X.] HISTOLOGY OF THE PLACENTA. 363
DIAGRAMMATIC REPRESENTATIONS OF THE MINUTE STRUCTURE
OF THE PLACENTA. (From Turner.)
F. the foetal ; M. the maternal placenta ; e. epithelium of cho-
rion ; e'. epithelium of maternal placenta ; d. foetal blood-
vessels ; d'. maternal blood-vessels ; v. villus.
I. Placenta in its most generalized form. II. Structure of
placenta of a Pig. III. Of a Cow. IV. Of a Fox. V. Of a
VI. Structure of placenta of a Sloth. On the right side of
the figure the flat maternal epithelial cells are shewn in situ.
On the left side they are removed, and the dilated maternal vessel
with its blood-corpuscles is exposed.
VII. Structure of Human placenta. In addition to the let-
ters already referred to, ds, ds. represents the decidua serotina of
the placenta ; t, t. trabeculae of serotina passing to the foetal villi ;
ca. curling artery ; up. utero-placental vein ; x. a prolongation of
maternal tissue on the exterior of the villus outside the cellular
layer e', which may represent either the endothelium of the
maternal blood-vessel or delicate connective tissue belonging to
the serotina, or both. The layer e' represents maternal cells
derived from the serotina. The layer of foetal epithelium cannot
be seen on the villi of the fully-formed human placenta.
In the human placenta (VII.), as in that of Apes,
the greatest modification is found. Here the maternal
vessels have completely lost their capillary form, and
have become expanded into large freely communicating
sinuses (d f ). In these sinuses the foetal villi hang for
the most part freely, though occasionally attached to
their walls by strands of tissue (t). In the late stages
of fcetal life there is only one epithelial layer (e} be-
tween the maternal and fcetal vessels, which closely
invests the fcetal villi, but is part of the uterine tissue.
In the foetal villi the vessels retain their capillary form.
364< EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. XI.
Evolution of the placenta. Excluding the mar-
supials whose placentation is not really known, the
arrangement of the foetal membranes of the Rabbit is
the most primitive observed. In this type the allantois
and yolk-sac both function in obtaining nutriment
from the mother ; and the former occupies only a small
discoidal area of the subzonal membrane. In all higher
types the allantois gradually spreads out over the whole
inner surface of the subzonal membrane and its im-
portance increases ; while that of the yolk-sac as a nu-
tritive organ decreases. In the diffuse type of placenta
simple villi are present over nearly the whole surface of
the chorion. In the remaining types the villi become
more complicated and restricted to a smaller area
(meta-discoidal, zonary, &c.) of the chorion ; though in
the early stages they are more scattered and simpler,
in some cases occupying nearly the whole surface of the
chorion. It therefore seems probable that the placenta
of Man has been derived not directly from the discoidal
placenta of the Rabbit, but from the diffuse placenta
such as is seen in the Lemurs, etc., and that generally
the zonary, cotyledonary, &c. types of placenta have
been derived from the diffuse by a concentration and
increase in the complexity of the fcetal villi.
THE DEVELOPMENT OF THE ORGANS IN MAMMALIA.
IN chap, X. we have described the early stages and
general development of the mammalian embryo. In
the present chapter we propose to examine the for-
mation of such mammalian organs as differ in their
development from those of the chick. This will not be
a work of any considerable extent, as in all essential
points the development of the organs in the two groups
is the same. They will be classified according to the
germinal layers from which they originate.
THE ORGANS DERIVED FROM THE EPIBLAST.
Hairs are formed in solid processes of the deep
(Malpighian) layer of the epidermis, which project into
the subjacent dermis. The hair itself arises from a
cornification of the cells of the axis of one of the above
processes ; and is invested by a sheath similarly formed
from the more superficial epidermic cells. A small
papilla of the dermis grows into the inner end of the
epidermic process when the hair is first formed. The
366 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
first trace of the hair appears close to this papilla, but
soon increases in length, and when the end of the hair
projects from the surface, the original solid process of
the epidermis becomes converted into an open pit, the
lumen of which is filled by the root of the hair.
The development of nails has been already described
on p. 283.
Glands. The secretory part of the various glandular
structures belonging to the skin is invariably formed
from the epidermis. In Mammalia it appears that
these glands are always formed as solid ingrowths of the
Malpighian layer. The ends of these ingrowths dilate
to form the true glandular part of the organs, while the
stalks connecting the glandular portions with the sur-
face form the ducts. In the case of the sweat-glands
the lumen of the duct becomes first established ; its
formation is inaugurated by the appearance of the
cuticle, and appears first at the inner end of the duct
and thence extends outwards. In the sebaceous glands
the first secretion is formed by a fatty modification of
the whole of the central cells of the gland.
The muscular layer of the secreting part of the
sweat-glands is said to be formed from a modification of
the deeper layer of the epidermic cells.
The mammary glands arise in essentially the same
manner as the other glands of the skin. The glands of
each side are formed as a solid bud of the Malpighian
layer of the epidermis. From this bud processes sprout
out, each of which gives rise to one of the numerous
glands of which the whole organ is formed.
XII.] THE HIND BRAIN. 367
The central nervous system.
The development of the spinal cord in Mammals
differs in no important respects from that of the chick,
and we have nothing to add to the account we have
already given of its general development and histoge-
nesis in that animal. The development of the brain
however will be described at greater length, and some
additional facts relative to the development of the
Avian brain will be mentioned.
The first differentiation of the brain takes place in
Mammalia before the closure of the medullary folds,
and results as in the chick in the formation of the three
cerebral vesicles, the fore-, mid- and hind-brain (Fig.
106, B). A cranial flexure precisely resembling that of
the chick soon makes its appearance.
The hind brain early becomes divided into two
regions, the rudimentary medulla oblongata and cere-
The posterior section, the medulla, undergoes changes
of a somewhat complicated character. In the first place
its roof becomes very much extended and thinned
out. At the raphe, where the two lateral halves
of the brain originally united, a separation, as it were,
takes place, and the two sides of the brain become
pushed apart, remaining united by only a very thin
layer of nervous matter, consisting of a single row of
flattened cells (Fig. 40). As a result of this peculiar
growth in the brain, the roots of the nerves of the two
sides, which were originally in contact at the dorsal
summit of the brain, become carried away from one
another, and appear to rise at the sides of the brain.
368 DEVELOPMENT OF OEGANS IN MAMMALIA. [CHAP,
The thin roof of the fourth ventricle thus formed
is somewhat rhomboidal in shape.
At a later period the blood-vessels of the pia
mater form a rich plexus over the anterior part of
this thin roof which becomes at the same time some-
what folded. The whole structure is known as the
tela vasculosa or choroid plexus of the fourth ventricle
(Fig. 119, chd 4). The floor of the whole hind -brain
becomes thickened, and there very soon appears on its
outer surface a layer of longitudinal non-medullated
nerve-fibres, similar to those which first appear on the
spinal cord (p. 252). They are continuous with a similar
layer of fibres on the floor of the mid-brain, where
they constitute the crura cerebri. On the ventral floor
of the fourth ventricle is a shallow continuation of the
anterior fissure of the spinal cord.
Subsequently to the longitudinal fibres already spoken of,
there develope first the olivary bodies of the ventral side of the
medulla, and at a still later period the pyramids. The fasciculi
teretes in the cavity of the fourth ventricle are developed shortly
before the pyramids.
When the hind-brain becomes divided into two
regions the roof of the anterior part does not become
thinned out like that of the posterior, but on the con-
trary, becomes somewhat thickened and forms a band-
like structure roofing over the anterior part of thc k
fourth ventricle (Fig. 39 c6).
This is a rudiment of the cerebellum, and in all
Craniate Vertebrates it at first presents this simple
structure and insignificant size.
In Birds the cerebellum attains a very considerable
development (Fig. 119 cbl), consisting of a folded central
XII.] THE HIND-BKAIN. 369
lobe with an arbor vitse, into which the fourth ventricle
is prolonged. There are two small lateral lobes, ap-
parently equivalent to the flocculi.
In Mammalia the cerebellum attains a still greater
development. The median lobe or vermiform process
LONGITUDINAL SECTION THROUGH THE BRAIN OF A CHICK OF
TEN DAYS. (After Mihalkovics.)
hms. cerebral hemispheres ; alf. olfactory lobe ; alf^ olfactory
nerve ; ggt. corpus striatum ; oma. anterior commissure ;
did 3. choroid plexus of the third ventricle ; pin. pineal
gland ; cmp. posterior commissure ; trm. lamina terminalis ;
chm. optic chiasma ; inf. infundibulum ; hph. pituitary body ;
bgm. commissure of Sylvius (roof of iter a tertio ad quartum
ventriculum) ; vma. velum medullse anterius (valve of Vieus-
sens) ; cbl. cerebellum ; chd 4. choroid plexus of the fourth
ventricle ; obt 4. roof of fourth ventricle ; obi. medulla oblon-
gata ; pns. commissural part of medulla ; inv. sheath of
brain ; bis. basilar artery ; crts. internal carotid.
F. & B. 24
370 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
is first developed. In the higher Mammalia the lateral
parts constituting the hemispheres of the cerebellum
become formed as swellings at the sides at a consider-
ably later period; these are hardly developed in the
Monotremata and Marsupialia.
The cerebellum is connected with the roof of the mid-brain in
front and with the choroid plexus of the fourth ventricle behind
by delicate membranous structures, known as the velum me-
dullse anterius (valve of Yieussens) (Fig. 119 vma) and the velum
The pons Varolii is formed on the ventral side of the floor of
the cerebellar region as a bundle of transverse fibres at about the
same time as the olivary bodies. It is represented in Birds by
a small number of transverse fibres on the floor of the hind-brain
immediately below the cerebellum.
The mid-brain. The changes undergone by the
mid-brain are simpler than those of any other part of
the brain. It forms, on the appearance of the cranial
flexure, an unpaired vesicle with a vaulted roof and^
curved floor, at the front end of the long axis of the
body (Fig. 67, MB). It is at this period in Mammalia
as well as in Aves relatively much larger than in the
adult: its cavity is known as the iter a tertio ad
quartum ventriculum or aqueductus Sylvii.
The roof of the mid-brain is sharply constricted
off from the divisions of the brain in front of and
behind it, but these constrictions do not extend to the
In Mammalia the roof and sides give rise to two
pairs of prominences, the corpora quadrigemina.
These prominences, which are simply thickenings
not containing' any prolongations of the iter, become
XII.] THE FOKE-BKAIN. 371
first visible on the appearance of an oblique transverse
furrow, by which the whole mid-brain is divided into an
anterior and posterior portion. The anterior portion is
further divided by a longitudinal furrow into the two
anterior tubercles (nates) ; but it is not until later on
that the posterior portion is similarly divided longitu-
dinally into the two posterior tubercles (testes).
The floor of the mid -brain, bounded posteriorly by
the pons Varolii, becomes developed and thickened into
the crura cerebri. The corpora geniculata interna also
belong to this division of the brain.
Fore-brain. The early development of the fore-
brain in Mammals is the same as in the chick. It forms
at first a single vesicle without a trace of separate
divisions, but very early buds off the optic vesicles,
whose history is described with that of the eye. The
anterior part becomes prolonged and at the same time
somewhat dilated. At first there is no sharp boundary
between the primitive fore-brain and its anterior
prolongation, but there shortly appears a constriction
which passes from above obliquely forwards and down-
Of these two divisions the posterior becomes the
thalamencephalon, while the anterior and larger division
forms the rudiment of the cerebral hemispheres (Fig.
39 cer) and olfactory lobes. For a considerable period
this rudiment remains perfectly simple, and exhibits no
signs, either externally or internally, of a longitudinal
constriction dividing it into two lobes.
The thalamencephalon forms at first a simple
vesicle, the walls of which are of a nearly uniform thick-
ness and formed of the usual spindle-shaped cells.
372 DEVELOPMENT OF OEGANS IN MAMMALIA. [CHAP.
The cavity it contains is known as the third ventricle.
Anteriorly it opens widely into the cerebral rudiment,
and posteriorly into the ventricle of the mid-brain.
The* opening into the cerebral rudiment becomes the
foramen of Monro.
For convenience of description we may divide the
thalamencephalon into three regions, viz. (1) the floor,
(2) the sides, and (3) the roof.
The floor becomes divided into two parts: an an-
terior part, giving origin to the optic nerves, in which is
formed the optic chiasma ; and a posterior part, which
becomes produced into a prominence at first incon-
spicuous the rudiment of the infundibulum (Fig. 39 In).
This cornes in contact with the involution from the
mouth which gives rise to the pituitary body (Fig.
In Birds, although there is a close connection be-
tween the pituitary body and the infundibulum, there
is no actual fusion of the two. In Mammalia the case
is different. The part of the infundibulum which lies
at the hinder end of the pituitary body is at first a
simple finger-like process of the brain (Fig. 120 inf)\
but its end becomes swollen, and the lumen in this
part becomes obliterated. Its cells, originally similar to
those of the other parts of the nervous system, and even
containing differentiated nerve-fibres, partly atrophy
and partly assume an indifferent form, while at the
same time there grow in amongst them numerous
vascular and connective-tissue elements. The process
of the infundibulum thus metamorphosed becomes in-
separably connected with the true pituitary body, of
which it is usually described as the posterior lobe.
XII.] THE THALAMENCEPHALON. 373
In the later stages of development the unchanged
portion of the infundibulum becomes gradually pro-
longed and forms an elongated diverticulum of the
third ventricle, the apex of which is in contact with
the pituitary body (Fig. 120 hph).
The posterior part of the primitive infundibulum becomes the
corpus albicans, which is double in Man and the higher Apes ;
the ventral part of the posterior wall forms the tuber cinereum.
Laterally, at the junction of the optic thalami and infundibulum,
there are continued some of the fibres of the crura cerebri, which
arc probably derived from the walls of the infundibulum.
The sides of the thalamencephalon become very
early thickened to form the optic thalami, which con-
stitute the most important section of the thalamen-
cephalon. These are separated on their inner aspect
from the infundibular region by a somewhat S-shaped
groove, known as the sulcus of Monro, which ends in
the foramen of Monro. They also become secondarily
united by a transverse commissure, the grey or middle
commissure, which passes across the cavity of the third
The roof undergoes more complicated changes. It
becomes divided, on the appearance of the pineal gland
as a sm^all papilliform outgrowth (the development of
which is dealt with below), into two regions a longer
anterior in front of the pineal gland, and a shorter pos-
terior. The anterior region becomes at an early period
excessively thin, and at a later period, when the roof of
the thalamencephalon is shortened by the approach of
the cerebral hemispheres to the mid-brain, it becomes
(vide Fig. 120 did 3) considerably folded, while at the
same time a vascular plexus is formed in the pia mater
374 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP,
LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR
PART OP THE BRAIN OF AN EMBRYO RABBIT OF FOUR
CENTIMETRES. (After Mihalkovics.)
The section passes through the median line so that the cere-
bral hemispheres are not cut ; their position is however indicated
spt. septum lucidum formed by the coalescence of the inner walls
of part of the cerebral hemispheres ; cma. anterior com-
missure ; frx. vertical pillars of the fornix ; cat. genu of
corpus callosum ; trm. lamina terminalis ; hms. cerebral
hemispheres ; olf. olfactory lobes ; ad. artery of corpus
callosum ; fmr. position of foramen of Monro ; chd 3. choroid
plexus of third ventricle ; pin. pineal gland ; cmp. posterior
commissure ; bgm. lamina uniting the lobes of the mid-
brain ; chm. optic chiasma ; hph. pituitary body ; inf. infun-
dibulum ; pns. pons Varolii ; pde. cerebral peduncles ; agd.
iter a tertio ad quartum ventriculum.
XII.] THE PINEAL GLAND. 375
above it. On the accomplishment of these changes it
is known as the tela choroidea of the third ventricle.
In the roof of the third ventricle behind the pineal
gland there appear transverse commissural fibres, form-
ing a structure known as the posterior commissure,
which connects together the two optic thalarni.
The most remarkable organ in the roof of the thala-
mencephalon is the pineal gland, which is developed as
a hollow papilliform outgrowth of the roof, and is at
first composed of cells similar to those of the other
parts of the central nervous system (Fig. 120 pin). It
is directed backwards over the hinder portion of the
roof of the thalamencephalon.
In Birds (p. 116) the primitive outgrowth to form
the pineal gland becomes deeply indented by vascular
connective-tissue ingrowths, so that it assumes a den-
dritic structure (Fig. 119 pin). The proximal extremity
attached to the roof of the thalamencephalon soon
becomes solid and forms a special section, known as
the infra-pineal process. The central lumen of the
free part of the gland finally atrophies, but the branches
still remain hollow. The infra-pineal process becomes
reduced to a narrow stalk, connecting the branched
portion of the body with the brain.
In Mammalia the development of the pineal gland
is generally similar to that of Birds. The original out-
growth becomes branched, but the follicles or lobes to
which the branching gives rise eventually become solid
(Fig. 120 pin). An infra-pineal process is developed
comparatively late, and is not sharply separated from
the roof of the brain.
No satisfactory suggestions have yet been offered as
376 DEVELOPMENT OF OKGANS IN MAMMALIA. [CHAP.
to the nature of the pineal gland. It appears to possess
in all forms an epithelial structure, but, except at the
base of the stalk (infra-pineal process) in Mammalia, in
the wall of which there are nerve-fibres, no nervous
structures are present in it in the adult state.
The cerebral hemispheres. It will be convenient
to treat separately the development of the cerebral
hemispheres proper, and that of the olfactory lobes.
In the cerebral rudiment two parts may be dis-
tinguished, viz. the floor and the roof. The former gives
rise to the ganglia at the base of the hemispheres, the
corpora striata, the latter to the hemispheres proper.
The first change which takes place consists in the
roof growing out into two lobes, between which a shallow
median constriction makes "its appearance (Fig. 121).
DIAGRAMMATIC LONGITUDINAL HORIZONTAL SECTION THROUGH
3.v. third ventricle ; Iv. lateral ventricle ; It. lamina terminalis ;
ce. cerebral hemisphere ; op. th. optic thalamus.
XII.] THE CEREBRAL HEMISPHERES. 377
The two lobes thus formed are the rudiments of the
two hemispheres. The cavity of each of them opens
by a widish aperture into a cavity at the base of the
cerebral rudiment, which again opens directly into the
cavity of the third ventricle (3 v). The Y-shaped aper-
ture thus formed, which leads from the cerebral hemi-
spheres into the third ventricle, is the foramen of
Monro. The cavity (Iv) in each of the rudimentary
hemispheres is a lateral ventricle. The part of the
cerebrum which lies between the two hemispheres, and
passes forwards from the roof of the third ventricle
round the end of the brain to the optic chiasma below,
is the rudiment of the lamina terminalis (Figs. 121 It
and 123 trm). Up to this point the development of
the cerebrum is similar in all Vertebrata, and in some
forms it practically does not proceed much further.
The cerebral hemispheres undergo in Mammalia the
most complicated development. The primitive un-
paired cerebral rudiment becomes, as in lower Ver-
tebrates, bilobed, and at the same time divided by the
ingrowth of a septum of connective tissue into two
distinct hemispheres (Figs. 125 and 124 / and 122 i).
From this septum is formed the falx cerebri and other
The hemispheres contain at first very large cavities,
communicating by a wide foramen of Monro with the
third ventricle (Fig. 124). They grow rapidly in size,
and extend, especially backwards, and gradually cover
the thalamencephalon and the mid-brain (Fig. 122 i,f).
The foramen of Monro becomes very much narrowed
and reduced to a mere slit.
The walls are at first nearly uniformly thick, but
378 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
BRAIN OF A THREE MONTHS' HUMAN EMBRYO : NATURAL SIZE.
1. From above with the dorsal part of hemispheres and mid-
brain removed ; 2. From below. /. anterior part of cut wall
of the hemisphere ; /'. cornu ammonis ; tho. optic thalamus ;
cst. corpus striatum ; to. optic tract ; cm. corpora mammil-
laria ; p. pons Yarolii.
the floor becomes thickened on each side, and gives rise
to the corpus striatum (Figs. 124 and 125 st}. The
corpus striatum projects upwards into each lateral ven-
tricle, and gives to this a somewhat semilunar form, the
two horns of which constitute the permanent anterior
and descending cornua of the lateral ventricles (Fig. 126
With the further growth of the hemisphere the cor-
pus striatum loses its primitive relations to the de-
scending cornu. The reduction in size of the foramen
of Monro above mentioned is, to a large extent, caused
by the growth of the corpora striata.
The corpora striata are united at their posterior
border with the optic thalami. In the later stages of
development the area of contact between these two
pairs of ganglia increases to a large extent (Fig. 125),
THE CORPORA STRIATA,
and the boundary between them becomes somewhat
obscure, so that the sharp distinction which exists
in the embryo between the thalamencephalon and
cerebral hemispheres becomes lost.
TRANSVERSE SECTION THROUGH THE BRAIN OF A RABBIT
FIVE CENTIMETRES. (After Mihalkovics.)
The section passes through nearly the posterior border of the
septum lucidum, immediately in front of the foramen of Monro.
hms. cerebral hemispheres ; cal. corpus callosum ; amm. cornu
ammonis (hippocampus major) ; cms. superior commissure
of the cornua ammonis ; spt. septum lucidum ;/r#2. anterior
pillars of the fornix ; cma. anterior commissure ; trm. lamina
terminalis ; sir. corpus striatum ; Itf. nucleus lenticularis
of corpus striatum ; vtr 1. lateral ventricle ; vtr 3. third
ventricle ; ipl. slit between cerebral hemispheres.
380 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
The outer wall of the hemispheres gradually thick-
ens, while the inner wall becomes thinner. In the
latter, two curved folds, projecting towards the interior
of the lateral ventricle, become formed. These folds
extend from the foramen of Monro along nearly the
whole of what afterwards becomes the descending cornu
of the lateral ventricle. The upper fold becomes the
hippocampus major (cornu ammonis) (Figs. 123 amm,
124 and 125 h, and 126 am).
The wall of the lower fold becomes very thin, and a
vascular plexus, derived from the connective-tissue
septum between the hemispheres, and similar to that of
the roof of the third ventricle, is formed outside it. It
constitutes a fold projecting into the cavity of the
lateral ventricle, and together with the vascular con-
nective tissue in it gives rise to the choroid plexus of
the lateral ventricle (Figs. 124 and 125 pi).
It is clear from the above description that a marginal
fissure leading into the cavity of the lateral ventricle
does not exist in the sense often implied in works on
human anatomy, since the epithelium covering the
choroid plexus, and forming the true wall of the brain,
is a continuous membrane. The epithelium of the
choroid plexus of the lateral ventricle is quite inde-
pendent of that of the choroid plexus of the third
ventricle, though at the foramen of Monro the roof of
the third ventricle is of course continuous with the
inner wall of the lateral ventricle (Fig. 124 s). The
vascular elements of the two plexuses form however a
The most characteristic parts of the Mammalian
cerebrum are the commissures connecting the two
THE CEREBRAL COMMISSURES.
hemispheres. These commissures are (1) the anterior
commissure, (2) the fornix, and (3) the corpus callosum,
the two latter being peculiar to Mammalia.
TRANSVERSE SECTION THROUGH THE BRAIN OP A SHEEP'S
EMBRYO OF 27 CM. IN LENGTH. (From Kolliker.)
The section passes through the level of the foramen of
st. corpus striatum ; m. foramen of Monro ; t. third ventricle ;
pi. choroid plexus of lateral ventricle ; /. falx cerebri ; th.
anterior part of optic thalamus ; ch. optic chiasma ; o. optic
nerve ; c. fibres of the cerebral peduncles ; h. cornu am-
monis ; p. pharynx ; sa. pre- sphenoid bone ; a. orbi to-
sphenoid bone ; s. points to part of the roof of the brain at
the junction between the roof of the third ventricle and
the lamina terminalis ; I. lateral ventricle.
382 DEVELOPMENT OF OEGANS IN MAMMALIA. [CHAP.
By the fusion of the inner walls of the hemispheres
in front of the lamina terminalis a solid septum is
formed, continuous behind with the lamina terminalis,
TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S
EMBRYO OF 2*7 CM. IN LENGTH. (From KOlliker.)
The section is taken a short distance behind the section
represented in Fig. 124, and passes through the posterior part of
the hemispheres and the third ventricle.
st. corpus striatum ; ih. optic thalamus ; to. optic tract ; t.
ventricle ; d. roof of third ventricle ; c. fibres of cerebr
peduncles ; c. divergence of these fibres into the walls of 1
hemispheres ; e. lateral ventricle with choroid plexus pi ;
h. cornu ammonis ; /. primitive falx ; am. alisphenoid ;
orbito-sphenoid ; sa. presphenoid ; p. pharynx ; mk. Meckel's
XII.] THE CORPUS CALLOSUM. 383
and below with the corpora striata (Figs. 120 and 123 spt).
It is by a series of differentiations within this septum,
the greater part of which gives rise to the septum luci-
dum, that the above commissures originate. In Man
there is a closed cavity left in the septum known as the
fifth ventricle, which has however no communication
with the true ventricles of the brain.
In this septum there become first formed, below and
behind, the transverse fibres of the anterior commissure
(Fig. 120 and Fig. 123 cma), while above and behind
these the vertical fibres of the fornix are developed
(Fig. 120 and Fig. 123 frx 2). The vertical fibres meet
above the foramen of Monro, and thence diverge back-
wards, as the posterior pillars, to lose themselves in the
cornu ammonis (Fig. 123 amm). Ventrally they are
continued, as the descending or anterior pillars of the
fornix, into the corpus albicans, and thence into the
optic thalami 1 .
The corpus callosum is not formed till after the
anterior commissure and fornix. It arises in the upper
part of the septum formed by the fusion of the lateral
walls of the hemispheres (Figs. 120 and 123 cal\ and
at first only its curved anterior portion the genu 01
rostrum is developed. This portion is alone found
in Monotremes and Marsupials. The posterior portion,
which is present in all the Monodelphia, is gradually
formed as the hemispheres are prolonged further back-
1 Recent observations tend to show that the anterior pillars of the
fornix end in the corpus albicans ; and that the fibres running from
the latter into the optic thalami are independent of the anterior
384 DEVELOPMENT OF OKGANS IN MAMMALIA. [CHAP.
Primitively the Mammalian cerebrum, like that of
the lower Vertebrata, is quite smooth. In some of the
Mammalia, Monotremata, Insectivora, etc., this condition
is retained nearly throughout life, while in the majority of
Mammalia a more or less complicated system of fissures
LATERAL VIEW OF THE BRAIN OF A CALF EMBRYO OF 5 CM.
The outer wall of the hemisphere is removed, so as to give a
view of the interior of the left lateral ventricle.
hs. cut wall of hemisphere ; st. corpus striatum ; am. hippo-
campus major (cornu ammonis) ; d. choroid plexus of lateral
ventricle ; fm. foramen of Monro ; op. optic tract ; in. in-
fundibulum ; mb. mid-brain ; cb. cerebellum ; IV. V. roof of
fourth ventricle ; ps. pons Varolii, close to which is the fifth
nerve with Gasserian ganglion.
is developed on the surface. The most important, and
first formed, of these is the Sylvian fissure. It arises at
the time when the hemispheres, owing to their growth
in front of and behind the corpora striata have assumed
somewhat the form of a bean. At the root of the
hemispheres the hilus of the bean there is formed a
XII.] HISTOGENESIS. 385
shallow depression which constitutes the first trace of
the Sylvian fissure. The part of the brain lying in this
fissure is known as the island of Reil.
The fissures of the cerebrum may be divided into two classes ;
(1) the primitive, (2) the secondary fissures. The primitive fissures
are the first to appear ; they owe their origin to a folding of the
entire wall of the cerebral vesicles. Many of them are transient
structures and early disappear. The most important of those
which persist are the hippocainpal, the parieto-occipital, the
calcarine (in Man and Apes) sulci and the Sylvian fissures.
The secondary fissures appear later, and are due to folds which
implicate the cortex of the hemispheres only.
The olfactory lobes. The olfactory lobes, or rhinen-
cephala, are secondary outgrowths of the cerebral hemi-
spheres, and contain prolongations of the lateral ven-
tricles, which may however be closed in the adult state ;
they arise at a fairly early stage of development from
the under and anterior part of the hemispheres (Fig.
Histogenetic changes. The walls of the brain are
at first very thin and, like those of the spinal cord, are
formed of a number of ranges of spindle-shaped cells.
In the floor of the hind- and mid-brain a superficial
layer of delicate nerve-fibres is formed at an early
period. This layer appears at first on the floor and
sides of the hind-brain, and almost immediately after-
wards on the floor and the sides of the mid-brain.
The cells internal to the nerve-fibres become differen-
tiated into an innermost epithelial layer lining the
cavities of the ventricles, and an outer layer of grey
The similarity of the primitive arrangement and
F. & B. 25
386 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM.
ch. cerebral hemispheres ; ol.v. olfactory vesicle ; off. olfactory
pit ; Sch. Schneiderian folds ; 1. olfactory nerve (the reference
line has been accidentally carried through the nerve so as to
appear to indicate the brain) ; pn. anterior prolongation of
histological characters of the parts of the brain behind
the cerebral hemispheres to those of the spinal cord is
very conclusively shewn by the examination of any good
series of sections. In both brain and spinal cord the
white matter forms a cap on the ventral and lateral
parts some considerable time before it extends to the
dorsal surface. In the medulla oblongata the white
matter does not eventually extend to the roof owing to
the peculiar degeneration which that part undergoes.
In the case of the fore-brain the walls of the hemi-
spheres become first divided (Kolliker) into a superficial
thinner layer of rounded elements, and a deeper and
thicker epithelial layer, and between these the fibres of
xii.] THE EYE. 387
the crura cerebri soon interpose themselves. At a
slightly later period a thin superficial layer of white
matter, homologous with that of the remainder of the
brain, becomes established.
The inner layer, together with the fibres from the
crura cerebri, gives rise to the major part of the white
matter of the hemispheres and to the epithelium lining
the lateral ventricles.
The outer layer of rounded cells becomes divided
into (1) a superficial part with comparatively few cells,
which, together with its coating of white matter, forms
the outer part of the grey matter, and (2) a deeper
layer with numerous cells, which forms the main mass
of the grey matter of the cortex.
The eyes. The development of the Mammalian eye
is essentially similar to that of the chick (ch. vi.) There
are however two features in its development which de-
serve mention. These are (1) the immense foetal develop-
ment of the blood-vessels of the vitreous humour and
the presence in the embryo of a vascular membrane sur-
rounding the lens, known as the membrana capsulo-
pujnllaris, (2) the absence of any structure comparable
to the pecten, and the presence of the arteria centralis
In the invagination of the lens (rabbit) a thin
layer of mesoblast is carried before it, and is thus
transported into the cavity of the vitreous humour.
In the folding in of the optic vesicle which accom-
panies the formation of the lens the optic nerve is
included, and on the development of the cavity of the
vitreous humour an artery, running in the fold of
the optic nerve, passes through the choroid slit into the
388 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
cavity of the vitreous humour (Fig. 128 acr). The sides
of the optic nerve subsequently bend over, and com-
pletely envelope this artery, which then gives off
[>/*. n -
SECTION THROUGH THE EYE OF A RABBIT EMBRYO OF ABOUT
c. epithelium of cornea : 1. lens ; mec. mesoblast growing in from
the side to form the cornea ; rt. retina ; a.c.r. arteria cen-
tralis retinse ; of.n. optic nerve.
The figure shews (1) the absence at this stage of mesoblast
between the lens and the epiblast ; the interval between the
two has however been made too great ; (2) the arteria centralis
retinae forming the vascular capsule of the lens and continuous
with vascular structures round the edges of the optic cup.
XII.] MEMBRANA CAPSULO-PUPILLARIS. 389
branches to the retina, and becomes known as the
arteria centralis retince. It is homologous with the
arterial limb of the vascular loop projecting into the
vitreous humour in Birds.
Before becoming enveloped in the optic nerve tins
artery is continued through the vitreous humour (Fig.
128), and when it comes in close proximity to the lens
it divides into a number of radiating branches, which
pass round the edge of the lens, and form a vascular
sheath which is prolonged so as to cover the anterior
wall of the lens. In front of the lens they anastomose
with vessels, coming from the iris, many of which are
venous, and the whole of the blood from the arteria
centralis is carried away by these veins. The vascular
sheath surrounding the lens is the membrana capsulo-
pupillaris. The posterior part of it is either formed
simply by branches of the arteria centralis, or out
of the mesoblast cells involuted with the lens. The
anterior part of the vascular sheath is however enclosed
in a very delicate membrane, the membrana pupillaris,
continuous at the sides with the membrane of Descemet.
The membrana capsulo-pupillaris is simply a pro-
visional embryonic structure, subserving the nutrition
of the lens.
In many forms, in addition to the vessels of the
vascular capsule round the lens, there arise from the
arteria centralis retinae, just after its exit from the optic
nerve, provisional vascular branches which extend them-
selves in the posterior part of the vitreous humour.
Near the ciliary end of the vitreous humour they anas-
tomose with the vessels of the membrana capsulo-pu-
390 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
The choroid slit closes very early, and is not per-
forated by any structure homologous with the pecteri.
The only part of the slit which can be said to remain
open is that in which the optic nerve is involved ; in the
Centre of the latter is situated the arteria centralis
retinae as explained above. From this artery there
grow out the vessels to supply the retina, which however
are distinct from the provisional vessels of the vitreous
humour just described, the blood being returned from
them by veins accompanying the arteries. On the
atrophy of the provisional vessels the whole of the blood
of the arteria centralis passes into the retina.
Of the cornea, aqueous humour, eyelids and lacrymal
duct no mention need here be made, the account given in
Part I. being applicable equally to mammalian embryos.
The auditory organ. In Mammals, as we have
seen to be the case in the chick (chap, vi.), the auditory
vesicle is at first nearly spherical, and is imbedded in
the mesoblast at the side of the hind-brain. It soon
becomes triangular in section, with the apex of the tri-
angle pointing inwards and downwards. This apex
gradually elongates to form the rudiment of the cochlear
canal and sacculus hemisphericus (Fig. 129, GO). At
the same time the recessus labyrinthi (R.L) becomes
distinctly marked, and the outer wall of the main body
of the vesicle grows out into two protuberances, which
form the rudiments of the vertical semicircular canals
(V.E). In the lower forms (Fig. 132) the cochlear
process hardly reaches a higher stage of development than
that found at this stage in Mammalia.
The parts of the auditory labyrinth thus established
soon increase in distinctness (Fig. 130); the cochlear
XII.] THE MEMBRANOUS LABYRINTH.
TRANSVERSE SECTION OF THE HEAD OF A F(ETAL SHEEP
(16 MM. IN LENGTH) IN THE REGION OF THE HIND-BRAIN.
HE. the hind-brain. The section is somewhat oblique, hence
while on the right side the connections of the recessus vestibuli
R.L.j and of the commencing vertical semicircular canal F.Z?.,
and of the ductus cochlearis CO., with the cavity of the primary
otic vesicle are seen : on the left side, only the extreme end of the
ductus cochlearis (7(7, and of the semicircular canal V.B. are shewn.
Lying close to the inner side of the otic vesicle is seen the
cochlear ganglion GC ; on the left side the auditory nerve G' and
its connection N with the hind-brain are also shewn.
Below the otic vesicle on either side lies the jugular vein.
392 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
canal ((7(7) becomes longer and curved ; its inner and
concave surface being lined by a thick layer of columnar
epiblast. The recessus labyrinthi also increases in
length, and just below the point where the bulgings to
form the vertical semicircular canals are situated, there
is formed a fresh protuberance for the horizontal semi-
SECTION OF THE HEAD OP A FOETAL SHEEP 20 MM. IN
LENGTH. (After Bottcher.)
R. V. Recessus labyrinthi ; V.B. vertical semicircular canal ; HE.
horizontal semicircular canal ; C.C. cochlear canal ; G. coch-
XII.] THE MEMBRANOUS LABYRINTH. 393
circular canal. At the same time the central parts of
the walls of the flat bulgings of the vertical canals grow
together, obliterating this part of the lumen, but leaving
a canal round the periphery ; and, on the absorption of
their central parts, each of the original simple bulgings
of the wall of the vesicle becomes converted into a true
semicircular canal, opening at its two extremities into
the auditory vesicle. The vertical canals are first es-
tablished and then the horizontal canal.
Shortly after the formation of the rudiment of the
horizontal semicircular canal a slight protuberance be-
comes apparent on the inner commencement of the
cochlear canal. A constriction arises on each side of
the protuberance, converting it into a prominent hemi-
spherical projection, the sacculus hemisphericus (Fig.
The constrictions are so deep that the sacculus is
only connected with the cochlear canal on the one hand,
and with the general cavity of the auditory vesicle on
the other, by, in each case, a narrow short canal. The
former of these canals (Fig. 131 6) is known as the
At this stage we may call the remaining cavity of
the original otic vesicle, into which all the above parts
open, the utriculus.
Soon after the formation of the sacculus hemispheri-
cus, the cochlear canal and the semicircular canals
become invested with cartilage. The recessus labyrinthi
remains however still enclosed in undifferentiated meso-
Between the cartilage and the parts which it sur-
rounds there remains a certain amount of indifferent
394 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC
SHEEP 28 MM. IN LENGTH. (After Bottcher.)
D.M. dura mater; R. V. recessus labyrinthi ; H.V.B. posterior
vertical semicircular canal ; U. utriculus ; H.B. horizontal
XII.] THE MEMBRANOUS LABYRINTH. 395
semicircular canal ; b. canalis reunions ; a. constriction by
means of which the sacculus hemisphericus S.lt. is formed ;
/. narrowed opening between sacculus hemisphericus and
utriculus ; C.C. cochlea ; C.C 1 . lumen of cochlea ; K.K.
cartilaginous capsule of cochlea ; K.B. basilar plate ; C/i.
connective tissue, which is more abundant around the.
cochlear canal than around the semicircular canals.
As soon as they have acquired a distinct connective-
tissue coat, the semicircular canals begin to bo dilated
at one of their terminations to form the ampullae. At
about the same time a constriction appears opposite the
mouth of the recessus labyrinthi, which causes its open-
ing to be divided into two branches one towards the
utriculus and the other towards the sacculus hemispheri-
cus ; and the relations of the parts become so altered
that communication between the sacculus and utriculus
can only take place through the mouth of the recessus
labyrinthi (Fig. 132).
When the cochlear canal has come to consist of two
and a half coils, the thickened epithelium which lines
the lower surface of the canal forms a double ridge
from which the organ of Corti is subsequently de-
veloped. Above the ridge there appears a delicate
cuticular membrane, the membrane of Corti or mem-
The epithelial walls of the utricle, the saccule, the
recessus labyrinthi, the semicircular canals, and the
cochlear canal constitute together the highly complicated
product of the original auditory vesicle. The whole
structure forms a closed cavity, the various parts of
which are in free communication. In the adult the
396 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
fluid present in this cavity is known as the endo-
In the mesoblast lying between these parts and the
cartilage, which at this period envelopes them, lymphatic
spaces become established, which are partially de-
veloped in the Sauropsida, but become in Mammals
very important structures.
They consist in Mammals partly of a space sur-
rounding the utricle and saccule and called the vestibule,
into which open spaces surrounding the semicircular
canals, and partly of two very definite channels, which
largely embrace between them the cochlear canal. The
latter channels form the scala vestibuli on the upper side
of the cochlear canal and the scala tympani on the lower.
The scala vestibuli is in free communication with the
lymphatic cavity surrounding the utricle and saccule,
and opens at the apex of the cochlea into the scala tyrn-
pani. The latter ends blindly at the fenestra rotunda.
The fluid contained in the two scalse, and in the
remaining lymphatic cavities of the auditory labyrinth,
is known as perilymph.
The cavities just spoken of are formed by an absorp-
tion of parts of the embryonic mucous tissue between
the perichondrium and the walls of the membranous
The scala vestibuli is formed before the scala tympani,
and both scalse begin to be developed at the basal end
of the cochlea : the cavity of each is continually being
carried forwards towards the apex of the cochlear canal
by a progressive absorption of the mesoblast. At first
both scalse are somewhat narrow, but they soon increase
in size and distinctness.
XII.] THE COCHLEA. 397
The cochlear canal, which is often known as the
scala media of the cochlea, becomes compressed on the
formation of the scalse so as to be triangular in section,
with the base of the triangle outwards. This base is
only separated from the surrounding cartilage by a
narrow strip of firm mesoblast, which becomes the stria
vascularis, etc. At the angle opposite the base the coch-
lear canal is joined to the cartilage by a narrow isthmus
of firm material, which contains nerves and vessels. This
isthmus subsequently forms the lamina spiralis, separ-
ating the scala vestibuli from the scala tympani.
The scala vestibuli lies on the upper border of the
cochlear canal, and is separated from it by a very thin
layer of mesoblast, bordered on the cochlear aspect by
flat epiblast cells. This membrane is called the mem-
brane of Reissner. The scala tympani is separated from
the cochlear canal by a thicker sheet of mesoblast, called
the basilar membrane, which supports the organ of
Corti and the epithelium adjoining it. The upper ex-
tremity of the cochlear canal ends in a blind extremity
called the cupola, to which the two scalse do not for
some time extend. This condition is permanent in
Birds, where the cupola is represented by a structure
known as the lagena (Fig. 132, II. L). Subsequently
the two scalse join at the extremity of the cochlear
canal ; the point of the cupola still however remains in
contact with the bone, which has now replaced the
cartilage, but at a still later period the scala vestibuli,
growing further round, separates the cupola from the
adjoining osseous tissue.
Accessory auditory structures. The development
of the Eustachian tube, tympanic cavity, tympanic
398 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
DIAGRAMS OF THE MEMBRANOUS LABYRINTH. (From Gegen-
I. Fish. II. Bird. III. Mammal
U. utriculus ; S. sacculus ; US. utriculus and sacculus ; Cr.
canalis reuniens ; R. recessus labyrinthi ; UC. commence-
ment of cochlea ; 0. cochlear canal ; L. lagena ; K. cupola
at apex of cochlear canal ; V. csecal sac of the vestibulum of
the cochlear canal.
membrane and external auditory meatus resembles that
in Birds (p. 166). As in Birds two membranous fenestrse,
the fenestra ovalis and rotunda, in the bony inner wall of
the tympanic cavity are formed. The fenestra ovalis
opens into the vestibule, and is in immediate contiguity
with the walls of the utricle, while the fenestra rotunda
adjoins the scala tympani. In place of the columella of
Birds, three ossicles, the malleus, incus and stapes reach
across the tympanic cavity from the tympanic membrane
THE NASAL ORGAN.
to the fenestra ovalis. These ossicles, which arise
mainly from the mandibular and hyoid arches (vide
p. 403), are at first imbedded in the connective tissue in
the neighbourhood of the tympanic cavity, but on the
full development of this cavity, become apparently
placed within it, though really enveloped in the mucous
membrane lining it.
Nasal organ. In Mammalia the general formation
of the anterior and posterior nares is the same as in
Birds; but an outgrowth from the inner side of the
canal between the two openings arises at an early period ;
and becoming separate from the posterior nares and
provided with a special opening into the mouth, forms
the organ of Jacobson. The general relations of this
organ when fully formed are shewn in Fig. 133.
SECTION THROUGH THE NASAL CAVITY AND JACOBSON'S ORGAN.
*n. septum nasi ; en. nasal cavity ; J. Jacobson's organ ; d. edgs
of upper jaw.
400 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
The development of the cranial and spinal
nerves in Mammals is as far as is known essentially
the same as in the chick, for an account of which see
p. 123 et seq.
Sympathetic nervous system. The development
of the sympathetic system of both Aves and Mammalia
has not been thoroughly worked out. There is how-
ever but little doubt that in Mammalia the main por-
tion arises in continuity with the posterior spinal
The later history of the sympathetic system is inti-
mately bound up with that of the so-called supra-renal
bodies, the medullary part of which is, as we shall see
below, derived from the peripheral part of the sympa-
THE ORGANS DERIVED FROM MESOBLAST.
The vertebral column. The early development of
the perichordal cartilaginous tube and rudimentary
neural arches is almost the same in Mammals as in
Birds. The differentiation into vertebral and inter-
vertebral regions is the same in both groups; but instead
of becoming divided as in Birds into two segments
attached to two adjoining vertebrae, the intervertebral
regions become in Mammals wholly converted into the
intervertebral ligaments (Fig. 135 li). There are three
centres of ossification for each vertebra, two in the arch
and one in the centrum.
The fate of the notochord is in important respects
different from that in Birds. It is first constricted in
the centres of the vertebrae (Fig. 134) and disappears
there shortly after the beginning of ossification ; while in
the intervertebral regions it remains relatively uncon-
stricted (Figs. 134 and 135 c) and after undergoing
certain histological changes remains through life as part
of the nucleus pulposus in the axis of the intervertebral
ligaments. There is also a slight swelling of the noto-
chord near the two extremities of each vertebra (Fig.
135 c and c"}.
In the persistent vertebral constriction of the notochord
Mammals retain a more primitive and piscine mode of formation
of the vertebral column thai} tfye majority either of the Reptilia
LONGITUDINAL SECTION THROUGH THE VERTEBRAL COLUMN
OF AN EIGHT WEEKS' HUMAN EMBRYO IN THE THO-
RACIC REGION. (From Kolliker.)
v. cartilaginous vertebral body ; li. intervertebral ligament ;
The skull. Excepting in the absence of the inter-
orbital plate, the early development of the Mamma-
lian cranium resembles in all essential points that of
Aves, to our account of which on p. 235 et seq. we refer
F. & B. 26
402 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
LONGITUDINAL SECTION THROUGH THE INTERVERTEBRAL LIGA-
MENT AND ADJACENT PARTS OF TWO VERTEBRA FROM THE
THORACIC EEGION OF AN ADVANCED EMBRYO OF A SHEEP.
la. .ligamentum longitudinale anterius ; Ip. ligamentum long, pos-
terius ; li. ligamentum intervertebrale ; , k r . epiphysis of
vertebra ; w. and w f . anterior and posterior vertebrae ; c. in-
tervertebral dilatation of notochord ; c.' and c". vertebral di-
latation of notochord..
The early changes in the development of the visceral
arches and clefts have already been described, but the
later changes undergone by the skeletal elements of the
first two visceral arches are sufficiently striking to need
a special description.
XII.] MANDIBULAR AND HYOID ARCHES. 403
The skeletal bars of both the hyoid and mandibular
arches develop at first more completely than in any
of the other types above Fishes ; they are articulated to
each other above, while the pterygo-palatine bar is
The main features of the subsequent development
are undisputed, with the exception of that of the upper
end of the hyoid, which is still controverted. The
following is Parker's account for the Pig.
The mandibular and hyoid arches are at first very
similar, their dorsal ends being somewhat incurved, and
In a somewhat later stage (Fig. 136) the upper end
of the mandibular bar (mb), without becoming segmented
KMHHYO Pra, AX INCH AND A THIRD LONG ; SIDE VIEW OF
MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID
ARCH IS SEEN AS DISPLACED BACKWARDS AFTER SEGMEN-
TATION FROM THE INCUS. (From Parker.)
t'j. tongue ; mJc. Meckelian cartilage ; ml. body of malleus ; ml).
inanubrium or handle of the malleus ; tjy. tegmen tympani ;
?'. incus ; st. stapes ; i.hy. interhyal ligament ; st.h. stylohyal
cartilage ; h.h. hypohyal ; b.h. basibranchial ; th.h. rudiment
of first branchial arch ; la. facial nerve.
404 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
from the ventral part, becomes distinctly swollen, and
clearly corresponds to the quadrate region of other types.
The ventral part of the bar constitutes Meckel's carti-
The hyoid arch has in the meantime become seg-
mented into two parts, an upper part (i), which eventually
becomes one of the small bones of the ear the incus-
and a lower part which remains as the anterior cornu
of the hyoid (st.h). The two parts continue to be con-
nected by a ligament.
The incus is articulated with the quadrate end of
the mandibular arch, and its rounded head comes in
contact with the stapes (Fig. 136, sf) which is segmented
from the fenestra ovalis.
According to some authors the stapes is independently formed
from mesoblast cells surrounding a branch of the internal carotid
The main arch of the hyoid becomes divided into
a hypohyal (hJi) below and a stylohyal (st.h) above, and
also becomes articulated with the basal element of the
arch behind (bh).
In the course of further development the Meckelian
part of the mandibular arch becomes enveloped in a
superficial ossification forming the dentary. Its upper
end, adjoining the quadrate region, becomes calcified
and then absorbed, and its lower, with the exception of
the extreme point, is ossified and subsequently incorpo-
rated in the dentary.
The quadrate region remains relatively stationary in
growth as compared with the adjacent parts of the skull,
and finally ossifies to form the malleiis. The processus
XII.] THE AUDITORY OSSICLES. ' 405
gracilis of the malleus is the primitive continuation into
The malleus and incus are at first embedded in the
connective tissue adjoining the tympanic cavity, which
with the Eustachian tube is the persistent remains of
the hyomandibular cleft ; and externally to them a bone
known as the tympanic bone becomes developed so that
they become placed between the tympanic bone and the
periotic capsule. In late foetal life they become trans-
ported completely within the tympanic cavity, though
covered by a reflection of the tympanic mucous mem-
The dorsal end of the part of the hyoid separated
from the incus becomes ossified as the tympano-hyal,
and is anchylosed with the adjacent parts of the periotic
capsule. The middle part of the bar just outside the
skull forms the stylo-hyal (styloid process in man) which
is attached by ligament to the anterior cornu of the
hyoid (cerato-hyal). The tympanic membrane and ex-
ternal auditory meatus develop as in the chick (p. 166).
The ribs and sternum appear to develop in Mammals as in
Birds (p. 234).
The pectoral girdle, as in Birds (p. 234), arises as a con-
tinuous plate of cartilage, the coracoid element of which is how-
ever much reduced.
The clavicle in Man is provided with a central axis of car-
tilage, and its mode of ossification is intermediate between that of
a true cartilage bone and a membrane bone.
The pelvic girdle is formed in cartilage as in Birds, but in Man
at any rate the pubic part of the cartilage is formed independently
of the remainder. There are the usual three centres of ossification,
which unite eventually into a single bone the innominate bone.
The pubis and ischium of each side unite ventrally, so as com-
pletely to enclose the obturator foramen.
406 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
The skeleton of the limbs develops so far as is known as in
Birds, from a continuous mesoblastic blastema, within which the
corresponding cartilaginous elements of the limbs become dif-
The body cavity. The development of the body
cavity and its subsequent division into pericardia!
pleural and peritoneal cavities is precisely the same in
Mammalia as in Aves (p. 264 et seq.). But in Mam-
malia a further change takes place, in that by the for-
mation of a vertical partition across the body cavity,
known as the diaphragm, the pleural cavities, contain-
ing the lungs, become isolated from the remainder of
the body or peritoneal cavity. As shewn by their
development the so-called pleurae or pleural sacs are
simply the peritoneal linings of the anterior divisions
of the body cavity, shut off from the remainder of the
body cavity by the diaphragm.
The vascular system.
The heart. The two tubes out of which the heart
is formed appear at the sides of the cephalic plates,
opposite the region of the mid- and hind-brain (Fig.
107). They arise at a time when the lateral folds
which form the ventral wall of the throat are only just
becoming visible. Each half of the heart originates in
the same way as in the chick ; and the layer of the
splanchnic mesoblast, which forms the muscular wall for
each part (ahh). has at first the form of a half tube open
below to the hypoblast.
On the formation of the lateral folds of the splanchnic
walls, the two halves of the heart become carried inwards
XII.] ARTERIAL SYSTEM. 407
and downwards, and eventually meet on the ventral
side of the throat. For a short time they here remain
distinct, but soon coalesce into a single tube.
In Birds, it will be remembered, the heart at first has the
form of two tubes, which however are in contact in front. It
arises at a time when the formation of the throat is very much
more advanced than in Mammalia ; when in fact the ventral
wall of the throat is established as far back as the front end of
In the lower types the heart does not appear till the ventral
wall of the throat is completely established, and it has from the
first the form of a single tub".
It is therefore probable that the formation of the heart as two
cavities is a secondary mode of development, which has been
brought about by variations in the period of the closing in of the
wall of the throat.
The later development of the heart is in the main similar to
that of the chick (p. 256 et seq.).
The arterial system. The early stages of the
arterial system of Mammalia are similar to those in
Birds. Five arterial arches are formed, the three poste-
rior of which wholly or in part persist in the adult.
The bulbus arteriosus is divided into two (fig. 137
B), but the left fourth arch (e), instead of, as in Birds,
the right, is that continuous with the dorsal aorta, and
the right fourth arch (i) is only continued into the right
vertebral and right subclavian arteries.
The fifth pair of arches which is continuous with
one of the divisions of the bulbus arteriosus gives origin
to the two pulmonary arteries. Both these however are
derived from the arch on one side, viz. the left (fig. 137
B); whereas in Birds, one pulmonary artery comes from
the left and the other from the right fifth arch (fig.
408 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
The ductus Botalli of the fifth arch (known in Man
as the ductus arteriosus) of the side on which the
pulmonary arteries are formed, may remain (e.g. in Man)
as a solid cord connecting the common stem of the
pulmonary aorta with the systemic aorta.
The diagram, Fig. 137, copied from Rathke, shews
at a glance the character of the metamorphosis the
arterial arches undergo in Birds and Mammals.
DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE AR-
TERIAL ARCHES IN A BIRD A. AND A MAMMAL B.
(From Mivart after Kathke.)
A. a. internal carotid ; b. external carotid ; c. common carotid ;
d. systemic aorta ; e. fourth arch of right side (root of dorsal
aorta) / right subclavian ; g. dorsal aorta ; h. left subcla-
vian (fourth arch of left side) ; i. pulmonary artery ; Jc. and
I. right and left ductus Botalli of pulmonary arteries,
B; a.' internal carotid ; b. external carotid ; c. common carotid ;
d.' systemic aorta ; e. fourth arch of left side (root of dorsal
aorta) ; /. dorsal aorta ; g. left vertebral artery ; h. left sub-
clavian artery ; i. right subclavian (fourth arch of right
side) ; Jc. right vertebral ; I. continuation of right subcla-
vian ; m. pulmonary artery ; n. ductus Botalli of pulmonary
XII.] VENOUS SYSTEM. 409
In some Mammals both subclavians spring from
a trunk common to them and the carotids (arteria
anonyma) ; or as in Man and some other Mammals,
the left one arises from the systemic aorta just beyond
the carotids. Various further modifications in the origin
of the subclavians are found in Mammalia, but they
need not be specified in detail. The vertebral arteries
arise in close connection with the subclavians, whereas
in Birds they arise from the common carotids.
The venous system. In Mammals the same venous
trunks are developed in the embryo as in Birds (Fig.
138 A). The anterior cardinals or external jugulars
form the primitive veins of the anterior part of the
body, and the internal jugulars and anterior vertebrals
are subsequently formed. The subclavians (Fig. 138
A, s), developed on the formation of the anterior limbs,
also pour their blood into these primitive trunks. In
the lower Mammalia (Monotremata, Marsupialia, Insec-
tivora, some Rodentia, etc.) the two ductus Cuvieri
remain as the two superior venae cavse, but more usually
an anastomosis arises between the right and left in-
nominate veins, and eventually the whole of the blood
of the left superior cava is carried to the right side, and
there is left only a single superior cava (Fig. 138 B and
C). A small rudiment of the Jeft superior cava remains
however as the sinus coronarius and receives the coronary
vein from the heart (Figs. 138 C, cor and 139 cs).
The posterior cardinal veins form at first the only
veins receiving the blood from the posterior part of the
trunk and kidneys ; and on the development of the hind
limbs receive the blood from them also.
An unpaired vena cava inferior becomes eventually
410 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS
SYSTEM OF MAMMALS (MAN). (From Gegenbaur.)
j. jugular vein ; cs. vena cava superior ; s. subclavian veins ; c.
posterior cardinal vein ; v. vertebral vein ; az. azygos vein ;
cor. coronary vein.
A. Stage in which the cardinal veins have already disap-
peared. Their position is indicated by dotted lines.
B. Later stage when the blood from the left jugular vein is
carried into the right to form the single vena cava superior ; a
remnant of the left superior cava being however still left.
C. Stage after the left vertebral vein has disappeared ; the
right vertebral remaining as the azygos vein. The coronary vein
remains as the last remnant of the left superior vena cava.
developed, and gradually carries off a larger and larger
portion of the blood originally returned by the posterior
cardinals. It unites with the common stem of the
allantoic and vitelline veins in front of the liver.
At a later period a pair of trunks is established
bringing the blood from the posterior part of the cardinal
veins and the crural veins directly into the vena cava
inferior (Fig. 139, il). These vessels, whose development
has not been adequately investigated, form the common
DIAGRAM OF THE CHIEF VENOUS TRUNKS OF MAN.
cs. coronary sinus ; s. subclavian vein ; ji. internal jugular ;
Je. external jugular ; az. azygos vein ; ha. hemiazygos vein ;
c. Jotted line shewing previous position of cardinal veins ;
d. vena cava inferior ; r. renal veins ; il. iliac ; Ity. hypogas-
tric veins ; h. hepatic veins.
The dotted lines shew the position of embryonic vessels
aborted in the adult.
iliac veins, while the posterior ends of the cardinal veins
which join them become the hypogastric veins (Fig.
Posterior vertebral veins, similar to those of Birds,
are established in connection with the intercostal and
412 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
lumbar veins, and unite anteriorly with the front part
of the posterior cardinal veins (Fig. 138 A).
Upon the formation of the posterior vertebral veins,
and upon the inferior vena cava becoming more im-
portant, the middle part of the posterior cardinals be-
comes completely aborted (Fig. 139 c), the anterior and
posterior parts still persisting, the former as the con-
tinuations of the posterior vertebrals into the anterior
vena cava (az\ the latter as the hypogastric veins (%).
Though in a few Mammalia both the posterior verte-
brals persist, a transverse connection is usually established
between them, and the one (the right), becoming the
more important, constitutes the azygos vein (Fig. 139
az\ the persisting part of the left forming the hemi-
azygos vein (ha).
The remainder of the venous system is formed in the
embryo by the vitelline and allantoic veins, the former
being eventually joined by the mesenteric vein so as to
constitute the portal vein.
The vitelline vein is the first part of this system
established, and divides near the heart into two veins
bringing back the blood from the yolk-sac (umbilical
vesicle). The right vein soon however aborts.
The allantoic (anterior abdominal) veins are origin-
ally paired. They are developed very early, and at first
course along the still widely open somatic walls of the
body, and fall into the single vitelline trunk in front.
The right allantoic vein disappears before long, and the
common trunk formed by the junction of the vitelline
and allantoic veins becomes considerably elongated.
This trunk is soon envelop'ed by the liver, and later in
its passage through, gives off branches to, and also
XII.] SUPRA-RENAL BODIES. 413
receives branches from this organ near its anterior exit.
The main trunk is however never completely aborted, as
in the embryos of other types, but remains as the ductus
With the development of the placenta the allantoic
vein becomes the main source of the ductus venosus,
and the vitelline or portal vein, as it may perhaps be
now conveniently called, ceases to join it directly, but
falls into one of its branches in the liver.
The vena cava inferior joins the continuation of the
ductus venosus in front of the liver, and, as it becomes
more important, it receives directly the hepatic veins
which originally brought back blood into the ductus
venosus. The ductus venosus becomes moreover merely
a small branch of the vena cava.
At the close of foetal life the allantoic vein becomes
obliterated up to its place of entrance into the liver;
the ductus venosus becomes a solid cord the so-called
round ligament and the whole of the venous blood is
brought to the liver by the portal vein.
Owing to the allantoic (anterior abdominal) vein
having merely a foetal existence an anastomosis between
the iliac veins and the portal system by means of the
anterior abdominal vein is not established.
The supra-renal bodies. These are paired bodies
lying anterior to the kidneys and are formed of two
parts, (1) a cortical and (2) a medullary portion. They
first appear in the Rabbit on the 12th or 13th day of
gestation, and arise as masses of mesoblast cells lying
between the aorta and the mesentery and to one side of
the former. On the 14th day they are well marked,
and lying dorsal to them is another mass of cells which
414 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
is found to be continuous with the sympathetic nervous
On the 16th day processes from the sympathetic
mass enter the mesoblastic tissue and become trans-
formed into the medullary portion of the adult supra-
renal ; while the mesoblastic tissue gives rise to the
The urinogenital organs.
The history of these organs in Mammalia, excepting
so far as concerns the lower parts of the urinogenital
ducts, is the same as in the Chick.
The Wolffian body and duct first appear, and are
followed by the Miillerian duct and the kidney. The
exact method of development of the latter structures
has not been followed so completely as in the Chick;
and it is not known whether the peculiar structures
found. at the anterior end of the commencing Miillerian
duct in Aves occur in Mammalia.
The history of the generative glands is essentially
the same as in the Chick.
Outgrowths from a certain number of Malpighian
bodies in the Wolffian body are developed along the
base of the testis, and enter into connection with the
seminiferous stroma. It is not certain to what parts of
the testicular tubuli they give rise, but they probably
form at any rate the vasa recta and rete vasculosum.
Similarly intrusions from the Malpighian bodies make
their way into the ovary of the female, and give rise to
cords of tissue which may persist throughout life.
The vasa efferentia (coni vasculosi) appear to be
derived from the glandular tubes of part of the Wolffian
XII.] GENITAL CORD. 415
body. The Wolffian duct itself becomes in the male the
vas deferens and the convoluted canal of the epididy-
mis ; the latter structure except the head being entirely
derived from the Wolffian duct.
The functionless remains of the embryonic organs described
for the chick (p. 224) are found also in mammals.
The Miillerian ducts persist in the female as the
Fallopian tubes and uterus.
The lower parts of the urinogenital ducts are some-
what further modified in the Mammalia than the Chick.
The genital cord. The lower part of the Wolffian
ducts becomes enveloped in both sexes in a special cord
of tissue, known as the genital cord (Fig. 1 40 gc), within
the lower part of which the Mulleriaii ducts are also
enclosed. In the male the Miillerian ducts in this cord
atrophy, except at their .distal end where they unite to
form the uterus masculinus. The Wolffian ducts, after
becoming the vasa deferentia, remain for some time
enclosed in the common cord but afterwards separate
from each other. The seminal vesicles are outgrowths of
the vasa deferentia.
In the female the Wolffian ducts within the genital
cord atrophy, though rudiments of them are for a long
time visible or even permanently persistent. The lower
parts of the Miillerian ducts unite to form the vagina
and body of the uterus while the upper become the
horns of the uterus and the Fallopian tubes. The
junction commences in the middle and extends forwards
and backwards ; the stage with a median junction being
retained permanently in Marsupials.
The urinogenital sinus and external generative
organs. The dorsal part of the cloaca with the alimen-
416 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
tary tract becomes partially constricted off from the
ventral, which then forms a urinogenital sinus (Fig. 140
ug). In the course of development the urinogenital
DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT
AN EARLY STAGE. (After Allen Thomson ; from Quain's
The parts are seen chiefly in profile, but the Mlillerian and
Wolffian ducts are seen from the front.
3. ureter; 4. urinary bladder; 5. urachus ; ot. genital ridge
(ovary or testis) ; W. left Wolffian body ; x. part at apex
from which coni vasculosi are afterwards developed ; w.
Wolffian duct ; m. Miillerian duct ; gc. genital cord consist-
ing of Wolffian and Miillerian ducts bound up in a common
sheath ; i. rectum ; ug. urinogenital sinus ; cp. elevation
which becomes the clitoris or penis ; Is. ridge from which the
labia majora or scrotum are developed.
XII.] EXTERNAL GENERATIVE ORGANS. 417
sinus becomes, in all Mammalia but the Ornithodelphia,
completely separated from the intestinal cloaca, and the
two parts obtain separate external openings. The
ureters (Fig. 140, 3) open higher up than the other
ducts into the stalk of the allantois which here dilates
to form the bladder. That part of the stalk which con-
nects the bladder with the ventral wall of the body
constitutes the urachus, and loses its lumen before the
close of embryonic life. The part of the stalk of the
allantois below the openings of the ureters narrows to
form the urethra, which opens together with the Wolffi an
and Mullerian ducts into the urogenital cloaca.
In front of the urogenital cloaca there is formed
a genital prominence (Fig. 140 cp) with a groove con-
tinued from the urinogenital opening, and on each side a
genital fold (Is). In the male the sides of the groove on
the prominence coalesce together, embracing between
them the opening of the urinogenital cloaca, and the
prominence itself gives rise to the penis, along which the
common urinogenital passage is continued. The two
genital folds unite from behind forwards to form the
In the female the groove on the genital prominence
gradually disappears, and the prominence remains as the
clitoris, which is therefore the homologue of the penis :
the two genital folds form the labia majora. The urethra
and vagina open independently into the common uro-
THE ALIMENTARY CANAL AND ITS APPENDAGES.
It is convenient to introduce into our account of the
organs derived from the hypoblast, a short account of
F. & B. 27
418 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
certain organs connected with the alimentary canal
such as the mesentery, stomodaeum, etc., which are not
hypoblastic in origin.
The origin of the hypoblast, and the process of
folding by which the cavity of the mesenteron is
established have already been described. The mesen-
teron may be considered under three heads.
1. The anterior or respiratory division of the
mesenteron. The pharynx, thyroid body, Eustachian
tube, tympanic cavity, oesophagus, trachea, bronchi, lungs
and stomach are developed from this portion, and their
development in the Mammal so closely resembles that in
the Chick that it is unnecessary for us to add to the
account we have already given in the earlier part of this
This section of the alimentary canal, as in the Chick,
is distinguished in the embryo by the fact that its walls
send out a series of paired diverticula which meet the
skin, and, after perforation has been effected at the
regions of contact, form the visceral clefts.
2. The middle division of the mesenteron, from
which the liver and pancreas are developed, as in the
Chick, forms the intestinal and cloacal region and is at
first a straight tube. It remains for some time connected
with the yolk sack.
The Cloaca appears as a dilatation of the mesen-
teron which receives, as in Aves, the opening of the
allantois almost as soon as the posterior section of
the alimentary tract is established. The eventual
changes which it undergoes have already been dealt
with in connection with the urinogenital organs.
The intestine. The posterior part of this becomes
XII.] THE MESENTERY. 419
enlarged to form the large intestine, while the anterior
portion becoming very much elongated and coiled forms
the small intestine, and moreover gives rise anteriorly
to the liver and pancreas.
From the large intestine close to its junction with the small
intestine an outgrowth is developed, the proximal part of which
enlarges to form the ccecum, while the distal portion in Man
forms the vermiform appendix.
3. The postanal division of the mesenteron atro-
phies at an early period of embryonic life. In the Chick
and lower types it communicates for a short time with
the hind end of the neural canal.
Splanchnic mesoblast and mesentery. The mesen-
teron consists at first of a simple hypoblastic tube, which
however becomes enveloped by a layer of splanchnic
mesoblast. This layer, which is not at first continued
over the dorsal side of the mesenteron, gradually grows
in, and interposes itself between the hypoblast of the
mesenteron, and the organs above. At the same time
it becomes differentiated into two layers, viz. an outer
epithelioid layer which gives rise to part of the peritoneal
epithelium, and an inner layer of undifferentiated cells
which in time becomes converted into the connective
tissue and muscular walls of the mesenteron. The
connective tissue layers are first formed, while of the
muscular layers the circular is the first to make its
Coincidently with the differentiation of these layers
the connective tissue stratum of the peritoneum becomes
The mesentery is developed as in the Chick (p. 172).
In the thoracic region it is hardly if at all developed.
420 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP.
The primitive simplicity in the arrangement of the
mesentery is usually afterwards replaced by a more com-
plicated disposition, owing to the subsequent elongation
and consequent convolution of the intestine and stomach.
The layer of peritoneal epithelium on the ventral
side of the stomach is continued over the liver, and
after embracing the liver, becomes attached to the
ventral abdominal wall. Thus in the region of the liver
the body-cavity is divided into two halves by a mem-
brane, the two sides of which are covered by the peri-
toneal epithelium, and which encloses the stomach
dorsally and the liver ventrally. The part of the mem-
brane between the stomach and liver is narrow, and
constitutes a kind of mesentery suspending the liver
from the stomach : it is known to human anatomists as
the lesser omentum.
The part of the membrane connecting the liver with
the anterior abdominal wall constitutes the falciform or
suspensory ligament of the liver. It arises by a secondary
fusion, and is not a remnant of a primitive ventral
mesentery (vide p. 264).
The mesentery of the stomach, or mesogastrium,
enlarges in Mammalia to form a peculiar sack known as
the greater omentum.
The stomodseum. The anterior section of the per-
manent alimentary tract is formed, as in the Chick, by
an invagination of epiblast, constituting a more or less
considerable pit, with its inner wall in contact with the
blind anterior extremity of the mesenteron.
From the epiblastic liniog of this pit are developed
the pituitary body and the salivary as well as the other
XII.] THE TEETH. 421
DIAGRAM SHEWING THE DIVISION OF THE PRIMITIVE BUCCAL
CAVITY INTO THE RESPIRATORY SECTION ABOVE AND THE
TRUE MOUTH BELOW. (From Gegenbaur.)
p. palatine plate of superior maxillary process; m. permanent
mouth; n. posterior part of nasal passage; e. internasal
A palate grows inwards from each of the superior
maxillary processes (Fig. 141), which, meeting in the
middle line, form a horizontal septum dividing the front
part of the stomodaeum into a dorsal respiratory section,
containing the opening of the posterior nares, and a
ventral cavity forming the permanent mouth. These
two divisions open into a common cavity behind. This
septum on the development within it of an osseous
plate constitutes the hard palate. A posterior pro-
longation in which no osseous plate is formed consti-
tutes the soft palate. An internasal septum (Fig. 141 e)
may more or less completely divide the dorsal cavity
into two canals, continuous respectively with the two
The teeth are special products of the oral mucous
membrane. They are formed from two distinct organs,
viz. an epithelial cap and a connective tissue papilla,
422 DEVELOPMENT OF ORGANS IN MAMMALIA. [XII.
which according to most authors give rise to the enamel
and dentine respectively.
The proctodsBUm. The cloacal section of the ali-
mentary canal is placed in communication with the
exterior by means of a shallow epiblastic invagination
constituting the proctodseum.
PRACTICAL INSTRUCTIONS FOB STUDYING THE DE-
VELOPMENT OF THE CHICK.
I. A. Incubators.
OF all incubators, the natural one, i.e. the hen,
is in some respects the best. The number of eggs
which fail to develope is fewer than with an arti-
ficial incubator, and the development of monstrosi-
ties is rarer. A good sitter will continue to sit
for thirty or more days at least, even though the
eggs are daily being changed. She should never
be allowed to want for water, and should be well
supplied according to her appetite with soft food.
It is best to place the food at some little distance
from the eggs, in order that the hen may leave
the eggs when feeding. She will sit most per-
sistently in a warm, quiet, somewhat darkened
spot. When an egg is placed under her, the date
should be marked on it, in order that the duration
of its incubation may be exactly known. When
the egg is intended to remain for some time, e.g.
for seven days or more, the mark should be bold
and distinct, otherwise it will be rubbed off.
424 PRACTICAL DIRECTIONS. [APR
On the whole however we have found it more
convenient to use a good artificial incubator. We
have ourselves used with success two different
incubators. One made by the Cambridge Scientific
Instrument Company, and the other by Wiesnegg
of 64, Rue Gay-Lussac, Paris (Fig. 65 in his
catalogue for 1881). We have had the longest ex-
perience with the former, and have found it work
exceedingly well : having been able to hatch chicks
without more attention than now and then turning
over the eggs.
Both these incubators consist essentially of a
large water-bath fitted with a gas regulator. They
are both perfectly automatic and when once regu-
lated require no further attention.
The temperature within the incubator should
be maintained at from 37 to 40C. A rise above
40 is fatal ; but it may be allowed to descend to
35 or in the young stages lower, without doing
any further harm than to delay the development.
The products of the combustion of the gas
should be kept as much as possible from the eggs,
while ou supply of fresh air and of moisture is
Tolerably satisfactory results may be obtained with
an ordinary chemical double- jacketed drying water-bath,
thoroughly covered in with a thick coat of cotton wool
and flannel baize, and heated by a very small gas-jet.
If the vessel be filled with hot water, and allowed to cool
down to 40 or thereabouts, before the eggs are introduced,
a very small gas flame will be sufficient to maintain the
requisite temperature. A small pin-hole-nozzle, giving
with ordinary pressure an exceeding narrow jet of flame
about two inches high, is the most convenient. By turn-
ing the gas off or on, so as to reduce or increase the height
APP.] HARDENING EMBRYOS. 425
of the jet as required, a very steady mean temperature
may be maintained.
In the absence of gas, a patent night-light placed at a
proper distance below the bath may be made to answer
very well. When a body of water, once raised to the
necessary temperature, is thoroughly surrounded with
non-conducting material, a very slight constant amount of
heat will supply all the loss.
B. On preparing sections of the embryo.
a. Picric acid.
We find this reagent the most satisfactory
for hardening the chick and in most instances
Klein enberg's solution of picric acid is the
With 100 parts of water, make a cold
saturated solution of picric acid ; add to this
two parts of concentrated sulphuric acid or
nitric acid : filter and add to the filtrate three
times its bulk of water.
In this solution of picric acid 1 the embryo
must be placed and left for from 2 5 hours.
It should then be washed in alcohol of 30 p.c.
and placed in alcohol 50 p.c. for one hour.
From this it must be removed into alcohol
of 70 p.c. in which it should be left until
all the picric acid is extracted ; to facilitate
this the 70 p.c. alcohol should be frequently
changed : when free from picric the embryo
1 It is sometimes advantageous to add to this solution of picric
acid as much pure kreasote as it will dissolve (vide Kleinenberg,
'Development of Earthworm," Quarterly Journal of Mic. Sci. 1879).
426 PRACTICAL DIRECTIONS. [APP.
should be placed in 90 p.c. alcohol and kept
there until required for further use.
2sT.B. Hardened embryos should always be
kept in 90 p.c. spirit and only placed in abso-
lute before imbedding, or staining with haema-
Some histologists prefer to keep hardened tissues
in alcohol 70 p.c.
b. Corrosive sublimate.
Place the embryo in a large quantity of a
saturated aqueous solution of corrosive subli-
mate to which a few drops of glacial acetic acid
have been added, and allow it to remain for
half-an-hour 1 . It is necessary thoroughly to ex-
tract the corrosive sublimate from the cells of the
embryo ; to accomplish this, wash it thoroughly
with water for from 10 minutes to 3 hours ac-
cording to the size of the object. The washing
may be limited to frequent changes of water or
the embryo may be placed in a vessel through
which a continuous stream of water is kept
running. "When all the sublimate is removed,
place it in 50 p.c. alcohol acidulated with nitric
acid (half-a-dozen drops of acid to a 4 oz.
bottle of spirit) for five minutes. The preser-
vation of the embryo is completed by treating
it with 70 p.c. alcohol for twenty-four hours and
then keeping it in 90 p.c. alcohol. We have
not found that corrosive sublimate gives such
good results as picric acid in the case of chicks
and mammalian embryos.
1 If there is only a small quantity of acetic acid mixed with the
sublimate, a prolonged immersion will do the embryo no harm.
APP.] HARDENING EMBRYOS. 427
c. Osmic acid.
Osmic acid is a difficult reagent to use, but
when properly applied it gives most excellent
It should be used as a weak solution ('I to
5 p.c.). The object should be left in it until
it has acquired a light brown tint. The stronger
the solution the less time is required for the
production of this tint. It should then be
removed and placed in picro- carmine, which
arrests the action of the osmic and stains the
embryo. The time required for the picro-car-
mine staining must be determined by practice.
From the picro-carmine the object must be
washed in 70 p.c. spirit; and then placed in
90, or may be preserved directly in glycerine.
If it is desired to use other staining agents
(borax-carmine is good for some preparations),
the object must be removed from osmic into
water or weak spirit, thence through 50 into
70 p.c., stained, and passed through 70 to
90 p.c. spirit.
d. After using osmic it is well in some cases
(mammalian segmenting ova) to place the
object in Miiller's fluid for 2 or 3 days, after
which it may be preserved in glycerine or spirit.
Miiller's fluid is made by dissolving 25 grms.
of bichromate of potash and 10 grms. of sodic
sulphate in 1000 cc. of water.
e. With chromic acid.
The embryo must be immersed in a solution
of the strength of *1 p.c. for 24 hours. From
this it should be removed and placed in a stronger
428 PRACTICAL DIRECTIONS. [APP.
solution (-3 p.c.) for another 24 hours. If it
then appears sufficiently hard, it may be at
once placed in alcohol of 70 p.c., in which it
should remain for one day, and then be trans-
ferred to alcohol of 90 p.c.
f. Absolute alcohol has also been employed as
a hardening reagent, but is by no means so good
as the reagents recommended above.
The object of these so-called hardening reagents is
to kill the tissues with the greatest possible rapidity
without thereby destroying them. The subsequent
treatment with alcohol completes the hardening which
is only commenced by these reagents.
There is room for the exercise of considerable skill
in the use of alcohol, and this skill can only be acquired
by experience. A few general rules may however be
(1) Tissues should not, generally, be changed from water
or an aqueous solution of the first hardening reagent
into an alcoholic solution of too great strength, nor
should the successive solutions of alcohol used differ
too much in strength. The distortion produced by
the violence and inequality of the diffusion currents
is thus diminished. This general rule should be
remembered in transferring tissues from alcohol to
the staining agents and vice versa.
(2) The tissues should not be left too long (more than
one or two hours) in alcoholic solutions containing
less than 70 p.c. of alcohol.
(3) They should not be kept in absolute alcohol longer
than is necessary to dehydrate them (see B. 1, p. 426).
The alcoholic solutions we generally use contain 30,
50, 70, 90 p.c. of alcohol.
In most cases it will be found of advantage
to stain the embryo. The best method of doing
APR] STAINING EMBRYOS. 429
this is to stain the embryo as a whole, rather
than to stain the individual sections after they
have been cut.
We have found hsematoxylin and borax-
carmine the best reagents for staining embryos
as a whole.
a. With hsematoxylin.
The best solution of hsematoxylin, one for
which we are indebted to Kleinenberg, is made
in the following way.
(1) Make a saturated solution of crystallized cal-
cium chloride in 70 p. a alcohol, and add
alum to saturation.
(2) Make also a saturated solution of alum in 70
p.c. alcohol, and add 1 to 2 in the proportion
of 1 : 8.
(3) To the mixture of 1 and 2 add a few drops of
a saturated solution of hsematoxylin in ab-
(4) It is often the case that hsematoxylin solution
prepared in this way has not the proper
purple tint ; but a red tint. This is due to
acidity of the materials used. The proper
colour can be obtained by treating it with
some alkaline solution. "We have found it
convenient to use for this purpose a saturated
solution of sodium bi-carbonate in 70 p.c.
spirit. (The exact amount must be deter-
mined by experiment, as it depends upon the
amount of acid present.)
The embryo should be placed for some hours
in absolute alcohol, before staining with hse-
430 PRACTICAL DIRECTIONS. [APP.
matoxylin, and should be removed directly from
absolute into the haematoxylin.
The time required for staining varies with
the size of the object and the strength of the
staining fluid. Hsematoxylin will not stain if
the embryo is not quite free from acid.
If the embryo is stained too dark, it should
be treated with a solution of 70 p.c. alcohol
acidulated with nitric acid (*25 p.c. of acid)
until the excess of staining is removed; and in
all cases the hsematoxylin staining is improved
by treating the embryo with acidulated 70 p.c.
After staining the embryo must be well
washed in 70 and placed in 90 p.c. spirit.
b. With borax-carmine.
Make an aqueous solution of 2 to 3 p.c.
carmine and 4 p.c. borax, by heating: add an
equal volume of 70 p.c. alcohol, and let the
mixture stand for thirty-six hours; after which
Stain the object thoroughly by leaving it in
this solution for one or even two days; it will
attain a dull maroon colour : transfer it then to
acidulated alcohol (see a) until it becomes a
bright red, and afterwards keep it as before in
90 p.c. alcohol.
This staining solution permeates more tho-
roughly and uniformly a large object than does
hsematoxylin : therefore when a four or five day
chick is to be stained, borax-carmine is the best
staining reagent to use. Embryos that have
been preserved in corrosive sublimate will be
APP.] STAINING EMBRYOS. 431
found to stain more thoroughly in this than in
the hsematoxylin solution.
c. With carmine.
Beale's carmine or some alcoholic solution is
the best. Into this the embryo may be removed
directly from 90 p.c. alcohol, left for 24 hours,
and then placed again in alcohol until required.
d. With picro-carmine.
This reagent is useful as will be seen later
for staining mammalian segmenting ova and
very young blastoderms ; it is used with the
greatest success after hardening in osmic acid.
There are several methods of making picro-
carmiue, the following is the simplest, and we
have found it answer our purpose fairly well.
To a solution made up of 1 grm. of car-
mine 4 cc. of liquor ammonia and 200 cc. of
distilled water add 5 grms. of picric acid; agitate
the mixture for some minutes, and then decant,
leaving the excess of acid.
The decanted fluid must remain for several
days, being stirred up from time to time; even-
tually evaporated to dryness in a shallow vessel,
and to every 2 grms. of the residue add 100 cc.
of distilled water.
e. With alum carmine.
To make it, boil a strong aqueous solution of
ammonia-alum with excess of carmine for 10 to
20 minutes, filter, and dilute the filtrate until
it contains from 1 to 5 p.c. of alum. Add a
few drops of carbolic acid to prevent the growth
432 PRACTICAL DIRECTIONS. [APR
Well hardened tissues may be left in this
aqueous solution for 24 hours. It is especially
good for staining nuclei ; as a rule the staining
is not diffuse, but it is necessary after staining
to treat with acid alcohol (see a).
3. IMBEDDING AND CUTTING SECTIONS.
It is not possible to obtain satisfactory sec-
tions of embryos without employing some
method of imbedding, and using a microtome.
Many imbedding solutions and methods of cut-
ting sections have been used, but we find the
following far superior to any other. It combines
several advantages \ in the first place it renders
it comparatively easy to obtain, what is so
essential, a complete consecutive series of sec-
tions of the embryo ; and secondly, all the sec-
tions when mounted are in the same relative
position ; and the various parts of each section
retain their normal position with regard to
The substance we prefer for imbedding is
paraffin. As will be seen below it is necessary
to have at hand paraffins of various melting
points, according to the temperature of the
room at the time when the sections are cut.
It will be found most convenient to obtain
paraffins of the highest and lowest melting
points and to mix them together as experience
Place the stained embryo in absolute alco-
hol until completely dehydrated (two hours is
sufficient for small embryos) : and when ready
APP.] IMBEDDING. 433
to imbed soak it in turpentine 1 until it is com-
pletely saturated : and transfer it thence with as
little turpentine as possible to a dish of melted
In cases of very delicate tissues, it is better to use
chloroform instead of turpentine. The chloroform
should be carefully added by means of a pipette to the
absolute alcohol in which the tissue is placed. The
chloroform sinks to the bottom of the bottle or tube
and the embryo, which at first lies at the junction of the
two liquids, gradually sinks into the chloroform. When
this is accomplished, remove all the absolute with a
pipette and add pieces of solid paraffin to the chloroform.
Gently warm this on a water bath till all the chloroform
is driven off ; then imbed in the usual way.
Care must be taken that no more heat is
used than is necessary to melt the paraffin ; for
this purpose the paraffin should be warmed over
a water bath the temperature of which is kept
constant (from 50 to 60C. but not more than
A paraffin melting at 44C. is of the proper consistency
for cutting when the temperature of the room is 15C.
With care a porcelain evaporating dish and
a gas flame may be made to answer, but the
student is advised not to imbed without a
The embryo may be left in the paraffin two,
three or more hours, after which it is imbedded
by placing it along with the melted paraffin in
either a box. made by bending up the sides and
folding in the corners of a piece of stiff" paper,
or what is better, a box formed by two L-shaped
1 If the alcohol is not quite absolute kreasote should be used
instead of turpentine.
F. & B. 28
PRACTICAL DIRECTIONS, [APP.
pieces of lead, placed on a glass slide in such a
manner as to enclose a space, The latter is
preferable because the object can be placed
in any position required with great ease by
moving it with a hot needle, and the whole can
be cooled rapidly. It is advisable, at any rate at
first, to arrange the embryo so as to cut it into
When cool a block of paraffin is formed, in
the midst of which is the embryo.
Other imbedding agents have been used. The best
of these are, (1) pure cocoa butter ; (2) a mixture of
spermaceti and castor oil or cocoa butter (4 parts of
the former to one of the latter). With these imbedding
substances, it is generally necessary to moisten the razor,
either with olive oil or turpentine and ribbons of sec-
tions cannot be made (see b).
When the imbedding block is cold pare away
the edges, then gradually slice it away until the
end of the embryo is near the surface, and
place it in a microtome.
The microtome we are most accustomed to is
a ' sliding microtome' made by Jung of Heidel-
berg ; it gives excellent results. Recently how-
ever Messrs CaldweH and Threlfall have designed
an automatic microtome which has been used
with success at the Cambridge Morphological
Laboratory and promises to effect a great saving
of time and trouble in cutting sections (vide p. 471
and Proceedings of the Cambridge Phil. Soc. 1883).
A convenient small microtome is one made by
Zeiss of Jena (also by the Cambridge Scientific
Instrument Company), in which the object is
fixed and by means of a finely divided screw
APP.] CUTTING SECTIONS. 435
raised through a hole in a glass plate, across
which a razor held in the hand is pushed. We
will briefly describe the method of manipulation
for the small microtome, it will be found easily
applicable to Jung's sliding microtome.
The paraffin block is pared in such a manner
that the edge nearest to the operator and that
opposite to him are parallel. A dry razor is
then pushed upon the glass plate over the hole
through which the block of paraffin projects up-
wards, and a section cut which remains upon
the razor. Care must be taken that the edge of
the razor is parallel to the parallel edges of the
paraffin block. The block having been raised
by the screw, a second section is made in the
same way and on the same part of the razor as
the first ; in consequence of which, the first
section will be pushed backwards by the second.
Similarly each new section pushes backwards
those already made ; and a ribbon of sections
formed which, if the paraffin is of the right
consistency, will adhere firmly together.
Experience must teach the manipulator how
to mix the paraffin in such a manner that it is
neither too hard nor too soft ; if it is too hard,
the sections will not adhere together and will
curl up on the razor, if too soft they will
stick to the razor and be found to be creased.
When it is not possible to keep the temperature
of the room constant it will be found convenient
to use a hard paraffin, and when necessary to
raise the temperature by means of a lamp.
The paraffin should completely surround the
embryo and fill up all the spaces within it.
436 PRACTICAL DIRECTIONS. [APP.
c. Mounting sections.
When the sections are cut, place them in
rows on a slide prepared in the following manner.
Make a solution of white shellac in kreasote
by heating, and let it be of the consistency of
glycerine, or slightly more fluid. With a camel's
hair-brush paint a very thin and uniform layer
of this gum over the slide which must be clean
and dry, and while the gum is wet place the sec-
tions in rows upon it. Now place the slide on a
water bath which is heated up to the melting
point of the paraffin. The sections sink down
into the thin layer of shellac and kreasote, the
kreasote slowly evaporates and the shellac be-
coming hard fixes the section in the position in
which it was placed on the slide. When the
kreasote has been evaporated, pour turpentine
carefully upon the slide, this dissolves the pa-
raffin and clears the sections which may at once
be mounted in Canada balsam.
A turpentine or chloroform solution of Canada balsam
should be used.
This method of cutting ribbons of sections
was first introduced by Mr Caldwell, to whom
we are also indebted for the account given above
for mounting sections (vide Note B, p. 471).
The latter however is a modification and im-
provement of Dr Giesbrecht's method. (Zoolo-
gischer Anzeiger No. 92, 1881.)
C. Preservation of the embryo as a whole.
Chick embryos of the first or second day may be
easily preserved whole as microscopic objects. For
this purpose, the embryo, which has been preserved
APP.] OPENING THE EGG. 487
in the ordinary way (B, a) should be stained slightly,
dehydrated, soaked in oil of cloves until transparent
and mounted in balsam.
Whole embryos of a later date cannot be satis-
factorily preserved as microscopic objects.
PRACTICAL DIRECTIONS FOR OBTAINING AND STUDYING
II. Examination of a 36 to 48 hours 1 embryo.
The student will find it by far the best plan to begin
with the study of an embryo of this date. The manipu-
lation is not difficult ; and the details of structure are
sufficiently simple to allow them to be readily grasped.
Earlier embryos are troublesome to manage until some
experience has been gained; and the details of later
ones are so many as to render it undesirable to begin
A. Opening tlie Egg.
Take the egg warm from the hen or the incu-
bator, and place it (it does not matter in what posi-
tion, since the blastoderm will at this stage always
be found at the uppermost part of the egg) in a
small basin large enough to allow the egg to be
covered with fluid. It is of advantage, but not
necessary, to place at the bottom of the basin a
mould, e.g. a flat piece of lead with a concavity on
the upper surface, in which the egg may rest securely
without rolling. Pour into the basin so much of a
'75 per cent, solution of sodium chloride warmed to
38C. as will cover the egg completely. With a sharp
tap break through the shell at the broad end over
the air-chamber, and let out as much air as has
already been gathered there. Unless this is done,
438 PRACTICAL DIRECTIONS. [APP.
the presence of air in the air-chamber will cause the
broad end to tilt up. At this date there will be
very little air, but in eggs of longer incubation, in-
convenience will be felt unless this plan be adopted.
Instead of being broken with a blow, the shell
may be filed through at one point, and the opening
enlarged with the forceps; but a little practice will
enable the student to use the former and easier-
method without doing damage.
With a blunt pair of forceps, remove the shell
carefully bit by bit, leaving the shell-membrane
behind; begin at the hole made at the broad end,
and work over the upper part until about a third or
half of the shell has been removed.
Then with a finer pair of forceps remove the
shell-membrane; it will readily come away in strips,
torn across the long axis of the egg in a somewhat
spiral fashion. The yolk and embryo will now come
It is the practice of some simply to break the egg
across and pour the yolk and white together into a
basin, very much as the housewife does. We feel
sure, however, that the extra trouble of the method
we have given will be more than repaid by the
During this time, and indeed during the whole
period of the examination of the embryo in situ, the
basin and its contents must be maintained, either by
renewal of the salt solution, or by the basin being
placed on a sand-bath, at about 38C.
B. Examination of the blastoderm in situ.
This may be done with the naked eye, or with a
simple lens of low power. Observe :
APP.] REMOVAL OF THE EMBRYO. 439
1. Lying across the long axis of the egg, the pellucid
area, in the middle of which the embryo may be
obscurely seen as a white streak.
2. The mottled vascular area, with the blood-vessels
just beginning to be formed.
3. The opaque area spreading over the yolk with the
changes in the yolk around its periphery.
4. (With a simple lens), the contractions of the heart;
perhaps the outlines of the head of the embryo
may be detected.
C. Removal of the embryo.
Plunge one blade of a sharp fine pair of scissors
through the blastoderm, just outside the outer margin
of the vascular area, and rapidly carry the incision
completely round until the circle is complete, avoid
as much as possible any agitation of the liquid in the
With a little trouble, the excised blastoderm may
now be floated into a watch-glass, care being taken to
keep it as flat as possible. With a pair of forceps or
with a needle, aided by gentle shaking, remove the
piece of vitelline membrane covering the blastoderm.
If any yolk adheres to the blastoderm, it may with
a little gentle agitation easily be washed off. Some-
times it is of advantage to suck up the yolk with a
glass syringe, replacing the fluid removed with clean
('75 p.c.) salt solution.
The blastoderm should now be removed from the
watch-glass to a microscopic glass slide ; since it is
difficult in the former to prevent the edges of the
blastoderm from curling up.
440 PRACTICAL DIRECTIONS. [APP.
The transference may easily be effected, if both
the watch-glass and slide are plunged into a basin of
clean warm salt solution. With a little care, the
blastoderm can then be floated from the one to the
other, and the glass slide, having the blastoderm with
its upper surface uppermost spread flat upon it, very
gently raised out of the liquid.
A thin ring of putty may now be placed round
the blastoderm, a small quantity of salt solution
gently poured within the ring, and the whole covered
with a glass slide, which may be pressed down until
it is sufficiently close to the embryo. The presence
of any air-bubbles must of course be avoided.
Provided care be otherwise taken to keep the
embryo well covered with liquid, the putty ring and
the coverslip may be dispensed with. They are often
inconvenient, as when the embryo has to be turned
The object is now re&dy for examination with a
simple lens or with a compound microscope of low
objective. It is by far the best for the student to
begin at least with the simple lens. In order that
everything may be seen at its best, the slide should
be kept warmed to about 38, by being placed on a
D. Surface view of the transparent embryo
The chief points to be observed are :
1. The head-fold.
2. The indications of the amnion; especially the
false amnion, or outer amniotic fold.
APP.] SURFACE VIEW. 441
3. The neural tube : the line of coalescence of the
medullary folds, the first cerebral vesicle, the com-
mencing optic vesicles, the indications of the
second and third cerebral vesicles, the as yet open
medullary folds at the tail end.
4. The heart seen dimly through the neural tube; note
its pulsation if present.
5. The fold of the somatopleure anterior to the heart
(generally very faintly shewn).
G. The fold of the splanchnopleurt (more distinctly
seen) : the vitelline veins.
7. The mesoblastic somites.
8. Indications of the vitelline arteries.
9. The as yet barely formed tail-fold.
10. The commencing blood-vessels in the pellucid and
E. Surface view of the transparent embryo from
The coverslip must now be removed and the glass
slide again immersed in a vessel of clean salt solu-
tion. By gently seizing the extreme edge of the
opaque area with a pair of forceps, no difficulty will
be found in so floating the blastoderm, as to turn it
upside down, and thus to replace it on the slide with
the under surface uppermost.
The points which most deserve attention in this
view, are :
1. The heart : its position, its union with the vitelline
veins, its arterial end.
442 PRACTICAL DIRECTIONS. [APP.
2. The fold of the splanchnopleure marking the hind
limit of the gut ; the vitelline veins running along
3. The mesoblastic somites on each side of the neural
canal behind the heart; farther back still, the ver-
tebral plates not divided into somites.
F. The examination of the embryo as an opaque
This should never be omitted. Many points in
the transparent embryo only become intelligible after
the examination of it as an opaque object.
Having removed the putty ring and coverslip, if
previously used, allow the blastoderm so far to be-
come dry, that its edge adheres to the glass slide.
Care must of course be taken that the embryo itself
does not become at all dry. Place the glass slide
with the blastoderm extended flat on it, in a shallow
vessel containing a solution of picric acid (I. B.).
If the blastoderm be simply immersed by itself in
the picric acid solution, the edges of the opaque
area will curl up and hide much of the embryo. The
method suggested above prevents these inconveni-
The embryo thus hardened and rendered opaque
by immersion in the acid (a stay of 2 to 3 hours in
the solution will be sufficient) may be removed to a
watch-glass, containing either some of the solution, or
plain water, and examined with a simple lens, imder
a strong direct light. The compound microscope will
be found not nearly so advantageous for this purpose
as the simple lens. A piece of black paper placed
under the watch-glass, will throw up the lights and
APP.] SURFACE VIEW. 443
shadows of the embryo, with benefit. The watch-
glass should have a flat bottom; or a shallow flat
glass cell should be used instead.
a. Looking at the embryo from above, observe :
1. The head-fold ; the head distinctly projecting from
the plane of the blastoderm, and formed chiefly by
the forebrain and optic vesicles.
2. The elevation of the medullary canal, and the
indications of the side walls of the embryo.
3. The indications of the tail.
4. The Amnion partly covering the head. Tear it
open with needles. Observe its two folds.
b. Having turned the blastoderm upside down,
observe the following points, looking at the embryo
1. The hinder limit of the splanchnopleure in the
head-fold, marking the hind limits of the fore-
gut. The opaque folds now conceal the head almost
entirely from view.
2. The commencing tail-fold, and the shallow boat-
shaped cavity (of the alimentary canal) between it
and the head-fold.
The student should not fail to make sketches
of the embryo, both as a transparent, and as an
opaque object, seen from below as well as from
above. These sketches will be of great service to
him when he comes to study the sections of the
444 PRACTICAL DIRECTIONS. [APP.
G. The following transverse sections will perhaps be
the most instructive.
Manipulation as in I. B. 3.
1. Through the optic vesicles, shewing the optic
2. Through the hind-brain, shewing the auditory
3. Through the middle of the heart, shewing its re-
lations to the splanchnopleure and alimentary canal.
4. Through the point of divergence of the splanch-
nopleure folds, shewing the venous roots of the
5. Through the dorsal region, shewing the medullary
canal, mesoblastic somites and commencing cleavage
of the mesoblast.
6. Through a point where the medullary canal is still
open, shewing the mode in which its closing takes
Longitudinal sections should also be made and
compared with the transverse sections.
III. Examination of an Embryo of about 4850 hours.
A. Opening the egg as in II. A.
B. Examination of the blastoderm in situ.
1. Thejform of the embryo, which is much more dis-
tinct than at the earlier stage.
2. The beating of the heart.
3. The general features of the circulation.
APP.J TRANSPARENT EMBRYO. 445
C. Removal of the Embryo from the yolk, as in
D. Surface view of the transparent embryo from
1. General form of the embryo.
a. Commencing cranial flexure.
b. The tail and side folds.
2. Amnion. Notice the inner and outer (false amnion)
limbs and remove them with a needle. When the
amnion has been removed the features of the
embryo will be much more clearly visible.
3. The organs of sense.
a. Eye. Formation of the lens already nearly
b. Auditory involution, now a deep sac with a
narrow opening to the exterior.
4. The brain.
a. The vesicles of the fore-, mid-, and hind-brsiui.
b. The cerebral vesicle.
c. The cranial flexure taking place at the mid-
E. Transparent embryo from below.
Manipulation as in II. E.
1. The increase of the head-folds of the somatopleure
and splanchnopleure, especially the latter, and the
commencement of these folds at the tail.
446 PRACTICAL DIRECTIONS. [APP.
2. The now as-shaped heart ; for further particulars
vide Chap. iv.
3. The commencing 1st and 2nd visceral clefts arid
the aortic arches.
4. The circulation of the yolk sac, vide Fig. 36. Make
out all the points there shewn and ascertain
by examination that what have been called the
veins and arteries in that figure, are truly such.
F. The embryo as an opaque object,
Treatment as in. II. F.
FROM ABOVE :
Observe the amnion, which is a very conspicuous
object, and remove it with needles if not done pre-
viously. The external form of the brain and the
auditory sac appear very distinctly.
FROM BELOW :
Observe the nature of the head- and tail-folds,
which are much more easily understood from the
opaque than from the transparent embryos.
Observe also the alimentary canal, the widely
open hind end of the fore-gut, and the front end of
the as yet very short hind-gut.
Manipulation as in I. B. 3.
The more important sections to be observed, are
1 . Through optic lobes, shewing :
a. The formation of the lens.
b. The involution of the primary optic vesicle.
c. The constriction, especially from above, of the
APP.] THIRD DAY EMBRYO. 447
2. Through auditory sac, shewing :
a. Auditory sac still open.
b. The thin roof and thick sides of the hind-brain.
e. Closed alimentary canal.
3. Through dorsal region, shewing the general appear-
ance of a section of an embryo at this stage, which
should be compared with a similar section of the
It shews :
a. The commencement of the side folds; the ali-
mentary canal still however open below.
b. The "Wolffian duct lying close under the epiblast
on the outside of the mesoblastic somites.
c. The notochord with the aortse on each side.
IY. Examination of an Embryo at the end of the third
A. Opening the egg, as in II. A.
B. Examination of the blastoderm in situ.
1. The great increase of the vascular area both in size
and distinctness. The circulation is now better
seen in situ than after the blastoderm has been
2. That the embryo now lies completely on its left
side and that it is only connected with the yolk-sac
by a somewhat broad stalk.
448 PRACTICAL DIRECTIONS. [APP.
C. Removal of the embryo. See II. C.
It is now unnecessary to remove the whole of the
blastoderm with the embryo ; indeed it is better to
cut away the vascular area unless it is wanted for
D. Surface view of the transparent embryo.
Since the embryo now lies on its side we shall
not have to speak of the view from above and below.
The views from the two sides differ chiefly as to the
appearance of the heart.
The embryo (freed from the blastoderm and the
amnion) is to be floated on to a glass slide in the
usual way. It is necessary to protect it while under
examination, with a coverslip, which must not be
allowed to compress it. To avoid this, we have found
it a good plan to support the coverslip at one end
only, since by moving it about when thus supported,
a greater or less amount of pressure can be applied
at will to the object.
The details which can at this stage be seen in a
transparent embryo are very numerous and we re-
commend the student to try and verify everything
shewn in Fig.* 37. Amongst the more important and
obvious points to be noticed are
1. The increase of the cranial flexure and the body-
2. The condition of the brain. The mid-brain now
forms the most anterior point of the head.
The fore-brain consists of the inconspicuous
vesicle of the third ventricle and the two large
APR] OPAQUE EMBRYO. 449
The hind-brain consists of a front portion, the
cerebellum with a thickened roof; and a hinder
portion, the fourth ventricle with a very thin and
3. Organs of sense.
The eye especially is now in a very good state
to observe. The student may refer to Fig. 51,
and the description there given.
The ear-vesicle will be seen either just closing
or completely closed.
4. In the region of the heart attention must also be
paid to :
a. The visceral clefts.
b. The investing-mass, Le. the growth of mesoblast
taking place around the end of the notochord.
c. The condition of the heart.
5. In the region of the body the chief points to be
observed are :
a. The increase in the number of the somites.
b. The Wolfflan duct, which can be seen as a streak
along the outer side of the hinder somites.
c. The attantois, which is now a small vesicle lying
between the folds of the somatopleure and
splanchnopleure at the hind end of the body, but
as yet hardly projects beyond the body cavity.
E. The embryo as an opaque object.
Preparation as in II. F.
The general form of the embryo can be very satis-
factorily seen when it is hardened and examined as an
opaque object; but the most important points to be
F. & B. 29
450 PRACTICAL DIRECTIONS. [APP.
made out at this stage in the hardened specimens are
those connected with the visceral clefts and folds and
If the amnion has not been removed it will be
necessary to pick it completely away with needles.
Without further preparation a view of the visceral
folds and clefts may be obtained from the side ; but
a far more instructive view is that from below, in
order to gain which the following method may be
Pour a small quantity of melted black wax (made
by mixing together lampblack and melted wax) into
a watch-glass, using just enough to cover the bottom
of the glass. While still soft make a small depression
in the wax with the rounded end of a pen-holder or
handle of a paint-brush and allow the wax to cool.
In the meantime cut off the head of the hardened
embryo by a sharp clean transverse incision carried
just behind the visceral clefts, transfer it to the
watch-glass and cover it with water or spirit. By a
little manipulation the head of the embryo may now
be shifted into the small depression in the wax,
and thus be made to assume any required position.
It should then be examined with a simple lens
under a strong reflected light, and a drawing made
When the head is placed in the proper position,
the following points may easily be seen.
1. The opening of the mouth bounded below by the
first pair of visceral folds, and commencing to be
enclosed above by the now very small buds which
are the rudiments of the superior maxillary pro-
cesses. Compare Fig. 56.
FOURTH DAY EMBRYO. 451
2. The second and third visceral arches and clefts.
3. The nasal pits.
F. Sections. Manipulation as in I. B. 3.
The most important sections are :
1. Through the eyes in the three planes, vide Fig. 50,
A. B. C.
2. Through the auditory sac.
3. Through the dorsal region, shewing the general
changes which have taken place.
Amongst these, notice
a. The changes of the mesoblastic somites: the com-
mencing formation of the muscle -plates.
b. The position of the Wolffian duct and the forma-
tion of the germinal epithelium.
c. The aortce and the cardinal veins.
d. The great increase in depth and relative diminu-
tion in breadth of the section.
V. Examination of an Embryo of the Fourth Day.
A. Opening the egg, as in II. A.
Great care will be required not to injure the
embryo, which now lies close to the shell-membrane.
B. Examination in situ. Observe:
1. The now conspicuous amnion.
2. The allantois, a small, and as yet hardly vascular
vesicle, beginning to project from the embryo into
the space between the true and the false anmion.
3. The rapidly narrowing somatic stalk.
452 PRACTICAL DIRECTIONS. [APP.
C. Removal of the embryo, as in II. C. and IV. C.
The remarks made in the latter place apply with
still greater force to an embryo of the fourth and
D. Surface mew of the transparent embryo. For
manipulation, vide IV. D.
The points to be observed are :
1. The formation of the fifth, seventh, and ninth
To observe these, a small amount of pressure
2. The formation of the fourth visceral cleft, and the
increase in size of the superior maxillary process.
3. The formation of the nasal pits and grooves.
4. The great relative growth of the cerebral lobes and
the formation of the pineal gland from the roof of
the vesicle of the third ventricle.
5. The great increase in the investing mass.
6. The formation and growth of the muscle-plates,
which can now be easily seen from the exterior.
7. The allantois. Make out its position and mode of
opening into the alimentary canal.
E. The embryo as an opaque object. Manipulation
as II. F. For mode of examination vide
The view of the mouth from underneath, shewing
the nasal pit and grooves, the superior and inferior
maxillary processes and the other visceral folds and
clefts, is very instructive at this stage. Compare
APP.] TWENTY HOURS EMBRYO. 453
F. Sections. Manipulation as in I. B. 3.
The most important sections are,
1. Through the eyes.
2. Transverse section immediately behind the visceral
arches, shewing the origin of the lungs.
3. Transverse section just in front of the umbilical
stalk, shewing the origin of the liver.
4. Transverse section at about the centre of the
dorsal region, to shew the general features of the
fourth day. Compare Fig. 68.
Amongst the points to be noticed in this section, are
b. Spinal nerves and ganglia.
c. Wolffian duct and bodies.
d. Miiller's duct.
f. Commencing changes in the spinal cord.
5. Section passing through the opening of the allan-
tois into the alimentary canal.
For the points to be observed in embryos of
the fifth and sixth days, the student must consult
the chapters devoted to those days.
In the hardened specimens, especial attention
should be paid to the changes which take place in
the parts forming the boundaries of the mouth.
VI. Examination of a Blastoderm of 20 hours.
A. Opening the egg, as in II. A.
B. Examination in situ.
It will not be found possible to make out anything
very satisfactory from the examination of a blasto-
454 PRACTICAL DIRECTIONS. [APP.
derm in situ at this age. The student will however
not fail to notice the halones, which can be seen
forming concentric rings round the blastoderm.
C. Removal of the embryo.
Two methods of hardening can be adopted at
this age. One of these involves the removal of the
blastoderm from the yolk, as in II. C. In the other
case, the yolk is hardened as a whole. If the latter
method be employed, the embryo cannot be viewed
as a transparent object.
In the cases where the blastoderm is removed
from the yolk, the manipulation is similar to that
described under II. C, with the exception of more
care being required in freeing the blastoderm from
the vitelline membrane.
D. Surface view transparent, from above.
1. The medullary groove between the two medullary
folds, whose hind ends diverge to enclose between
them the end of the primitive groove.
2. The head-fold at the end of the medullary groove.
3. The one or two pairs of mesoblastic somites flanking
the medullary groove.
4. The notochord as an opaque streak along the floor
of the medullary groove.
E. Surface view transparent \ from below.
Same points to be seen as from above, but less
APP.] TWENTY HOUKS EMBRYO. 455
F. Embryo as an opaque object.
As an opaque object, whether the embryo is hard-
ened in situ or after being removed from the yolk,
the same points are to be seen as when it is viewed
as a transparent object, with the exception of the
notochord and mesoblastic somites (vide D). The
various grooves and folds are however seen with far
Two methods of hardening may be employed ;
(1) with the embryo in situ, (2) after it has been
To harden the blastoderm in situ the yolk must
be hardened as a whole. After opening the egg either
leave the yolk in the egg-shell or pour it out into a
Berlin capsule ; in any case freeing it as much as
possible from the white, and taking especial care to
remove the more adherent layer of white which im-
mediately surrounds the yolk.
Place it in picric acid or a weak solution of chromic
acid (first of '1 p.c. and then of '5 p.c.) with the
blastoderm uppermost and leave it in that position
for two or three days.
Care must be taken that the yolk does not roll
about ; the blastoderm must not be allowed to alter
its position : otherwise it may be hard to find it when
everything has become opaque. If at the end of the
second day the blastoderm is not sufficiently hard,
the strength of the solution, if chromic acid be used,
should be increased and the specimen left in it for
After it has become hardened by the acid, the
yolk should be washed with water and treated sue-
456 PRACTICAL DIRECTIONS. [APR
cessively with weak and strong spirit, vide I. B.
After it has been in the strong spirit (90 p.c.) for two
days, the vitelline membrane may be safely peeled off
and the blastoderm and embryo will be found in
situ. The portion of the yolk containing them must
then be sliced off with a sharp razor, and placed in
The staining, <fec. may be effected in the ordinary
If osmic acid, which we believe will be found
serviceable for these ear]y stages, is employed, it will
be necessary to remove the blastoderm from the yolk
before treating it with the reagent.
The following transverse sections are the most im-
portant at this stage :
1. Through the medullary groove, shewing
a. The medullary folds with the thickened meso-
b. The notochord under the medullary groove.
c. The commencing cleavage of the mesoblast.
2. Through the region where the medullary folds
diverge, to enclose the end of the primitive groove,
shewing the greatly increased width of the medul-
lary groove, but otherwise no real alteration in
the arrangement of the parts.
3. Through the front end of the primitive groove
with the so-called axis cord underneath it, while
on each side of it are still to be seen the medul-
4. Through the primitive groove behind this point,
shewing the typical characters of the primitive
APP.] UNINCUBATED BLASTODERM. 457
VII. Examination of an unincubated Blastoderm.
A. Opening the egg. Vide II. A.
B. Examination of the blastoderm in situ.
Observe the central white spot and the peripheral
more transparent portion of the blastoderm and the
halones around it.
C. Removal of the blastoderm. Vide VI. C.
With the unincubated blastoderm still greater care
is required in removal than with the 20 hours' blasto-
derm, and there is no special advantage in doing so
unless it is intended to harden it with osmic acid.
D. Surface view transparent from above.
Observe the absence of the central opacity.
E. Surface view transparent from underneath.
Nothing further to be observed than from above.
F. As an opaque object.
There is nothing to be learnt from this.
Manipulation as in VI. G.
The sections shew
a. The distinct epiblast.
b. The lower layer cells not as yet differentiated
into mesoblast and hypoblast.
c. The thickened edge of the blastoderm.
d. The segmentation cavity and formative cells.
458 PRACTICAL DIRECTIONS. [APP.
VIII. Examination of the process of Segmentation.
To observe the process of segmentation it will be
found necessary to kill a number of hens which are
laying regularly. The best hens lay once every 24
hours, and by observing the time they usually lay (and
they generally lay pretty regularly about the same
time), a fair guess may be made beforehand as to
the time the egg has been in the oviduct. By this
means a series of eggs at the various stages of seg-
mentation may usually be obtained without a great
unnecessary sacrifice of hens. For making sections,
the yolk must in all cases be hardened as a whole,
which may be done as recommended in VI. G.
Chromic acid is an excellent reagent for this and
it will be found very easy to make good sections.
In the sections especial attention should be paid,
1. To the first appearance of nuclei in the segments,
and their character.
2. To the appearance of the horizontal furrows.
3. As to whether new segments continue to be formed
outside the limits of the germinal disc, or whether
the fresh segmentation merely concerns the already
4. In the later stages, to the smaller central and
larger peripheral segments, both containing nuclei.
For surface views, the germinal disc, either
fresh or after it has been hardened, can be used.
In both cases it should be examined by a strong
reflected light. The chief point to be noticed is
the more rapid segmentation of the central than of
the peripheral spheres.
APP.] STUDY OF BLOOD-VESSELS. 459
IX. Examination of the later changes of the Embryo.
For the later stages, and especially for the deve-
lopment of the skull and the vascular system of the
body of the chick, it will be found necessary to dissect
the embryo. This can be done either with the fresh
embryo or more advantageously with embryos which
have been preserved in spirit.
If the embryos are placed while still living into
spirit a natural injection may be obtained. And such
an injection is the best for following out the arrange-
ment of the blood-vessels.
Sections of course will be available for study,
especially when combined with dissections.
X. Study of the development of the Blood-vessels.
Observations on this subject must be made with
blastoderms of between 30 40 hours. These are to
be removed from the egg, in the usual way (vide II.
A. and C.), spread out over a glass slip and examined
from below, vide II. E.
The blastoderm when under examination must be
protected by a coverslip with the usual precautions
against pressure and evaporation, and a hot stage
must also be employed.
Fresh objects so prepared require to be examined
with a considerable magnifying power (400 to 800
diameters). From a series of specimens between 30
and 40 hours old all the points we have mentioned
in Chapter iv. p. 92, can without much difficulty be
Especial attention should be paid in the earlier
specimens to the masses of nuclei enveloped in pro-
toplasm and connected with each other by proto-
460 PKACTICAL DIRECTIONS. [APP.
plasmic processes; and in the later stages to the
breaking up of these masses into blood corpuscles
and the conversion of the protoplasmic processes
into capillaries, with cellular walls.
Blastoderms treated in the following ways may
be used to corroborate the observations made on the
With gold chloride.
Immerse the blastoderm in gold chloride (-5 p.c.)
for one minute and then wash with distilled water
and mount in glycerine and examine.
By this method of preparation, the nuclei and
protoplasmic processes are rendered more distinct,
without the whole being rendered too opaque for
The blastoderm after the application of the gold
chloride should become a pale straw colour; if it
becomes in the least purple, the reagent has been
applied for too long a time.
With potassium bichromate.
Immerse in a 1 p.c. solution for one day and then
mount in glycerine.
With osmic acid.
Immerse in a *5 p.c. solution for half an hour and
then in absolute alcohol for a day, and finally mount
PKACTICAL DIRECTIONS FOR OBTAINING AND STUDYING
XI. Animals and breeding.
For class work the Rabbit is the most convenient
animal from which to obtain embryos, it will breed
APP.] MAMMALIAN SEGMENTING OVA. 461
freely in the early spring months of the year and will
give ample opportunity for the student to observe the
exact time when the female is covered. A number
of does should be kept together in a large pen, and
two or three bucks in separate small cages also placed
within the pen ; at the period of heat, the doe should
be temporarily placed with the buck and the exact
time of copulation noted, the age of the embryo
being calculated from that hour.
XII. Examination of segmenting ova.
It will be well to mention here that although
a doe may have been satisfactorily covered, embryos
are not always obtained from her. A superficial
examination of the ovaries will determine whether or
no fertilized ova are present. If ova have been
recently dehisced from the ovary, the Graafian follicles
from which they were discharged will be found to be
of a distinctly red colour. In case no such ' corpora
lutea ' as they are called are present further search is
A. To obtain ova from i to 60 hours old.
Cut open the abdomen from pubis to sternum,
and from the pubis round the thigh of each side, and
turn back the flaps of the body wall so formed.
Remove the viscera and observe below (dorsal) the
single median vagina, from the anterior end of which
the uterine horns diverge.
Observe at the anterior end of each uterine horn
a small much coiled tube, the oviduct (Fallopian
tube) near the anterior end of which a little below
the kidney lies the ovary. Cut out the uterus and
oviduct together and lay them in a small dissecting
462 PRACTICAL DIRECTIONS. [APP.
dish. Carefully stretch out the oviduct by cutting
the tissue which binds it, and separating it from
the uterus, taking care to obtain its whole length,
lay it upon a glass slide.
With the aid of a lens it is frequently possible to
distinguish the ovum or ova, through the wall of the
oviduct. In this case cut a transverse slit into the
lumen of the duct with a fine pair of scissors a little
to one side of an ovum ; press with a needle upon
the oviduct on the other side of the ovum, which will
glide out through the slit, and can be with ease trans-
ported upon the point of a small scalpel, or what is
better spear-headed needle. In case the ovum cannot
be distinguished in the oviduct by superficial obser-
vation, the latter must be slit up with a fine pair of
scissors, when it will easily be seen with the aid of an
ordinary dissecting lens.
B. Treatment of the ovum.
The ovum may be examined fresh in salt solution,
it is however more instructive when preserved and
stained in the following manner.
a. Immerse it in a \ p.c. solution of osmic acid for
5 or even 10 minutes, transfer it thence to
the picrocarmine solution described above (I).
After staining the ovum should then be washed
in distilled water and placed in a weak solu-
tion of glycerine in a watch-glass half gly-
cerine, half water. It should be allowed to
remain thus under a bell jar for several days
(7 to 14 or longer) in a warm room until the
water has evaporated. By this means shrinkage
and distortion are avoided, the glycerine becoming
APP.] EXAMINATION OF OVUM. 463
very gradually more and more dense. It should
be mounted in glycerine in which 1 p.c. formic
acid has been mixed to prevent fungoid growths.
Care must be taken that there is no pressure
upon the ovum this being insured by the inser-
tion of a couple of slips of paper one on each side
of the ovum under the cover glass.
b. Another method of preservation is used, but
does not appear to us so successful as the one
already described. It consists of an immersion
of the ovum for 5 minutes in i to J p.c. osmic
acid, subsequent treatment with Mtiller's fluid
for two or three days, and finally mounting in
C. Examination of the ovum.
The most instructive stages to observe are ova of
a. 18 hours old, when four segmentation spheres
will be observed.
b. 36 hours old when the segmentation is more
advanced and the spheres numerous.
The chief points to be noted are :
1. The number and size of the segmentation spheres;
in each of which, when treated as described in B. a.,
a large deeply stained nucleus will be visible. The
spheres themselves are also stained slightly.
2. The presence of one or two polar bodies on the
outer side of the segments in ova of not more than
48 hours old: these also are slightly stained.
3. The zona radiata immediately surrounding the
4. The thick albuminous coat, marked with con-
464 PRACTICAL DIRECTIONS. [APP.
D. The fully segmented ovum. 70 hours old.
The fully segmented ovum is found in the uterus
at its anterior end close to the place where the
oviduct opens into the uterus.
To obtain this stage the uterus must be slit open
and examined carefully with a dissecting lens : the
ovum will be seen as a somewhat opaque spot on the
glistening moist mucous epithelium of the uterus.
It may be treated in the manner described under
B. a., but the segments being closely pressed to-
gether their outlines are not rendered distinct by
this method. A more advantageous mode of treatment
is the following : wash the ovum rapidly in distilled
water, and place it in a 1 p.c. solution of silver
nitrate for about 3 minutes : then expose it to the
light in a dish of distilled water until it be tinged
a brown colour.
The brown colour is due to the reduction of the
silver, which takes place chiefly in the cement sub-
stance between the cells and thus defines very exactly
their size and shape. The ovum may now be treated
with glycerine and mounted as described in B.
The points to be observed are :
1. The division of the segmentation spheres into the
layers an outer layer of cubical hyaline cells, and
an inner of rounded granular cells.
2. The blastopore of van Beneden.
3. The presence of a thin layer of mucous outside
the concentrically ringed albuminous coat of the
APR] BLASTODERMIC VESICLE. 465
XIII. Examination of the blastodermic vesicle, 72 90 hours.
A. To obtain tJie embryo see XII. D.
B. Prepare the ovum either as in XII. B. or D.
or in picric acid see I. B. i.
C. Surface view, or in section see I. B. 3.
1. The great increase in size of the ovum and the
reduction in the thickness of the membranes.
2. The flattened layer of outer cells enclosing a cavity.
3. The rounded cells of the inner mass attached as a
lens-shaped mass to one side of the vesicle.
XIV. Examination of a blastodermic vesicle of 7 days,
in which the embryonic area and primitive streak are
A. To obtain the embryo.
On opening the body cavity the uterus will be
found to be uniformly swollen and very vascular.
Remove the uterus and open it carefully with
fine scissors along the free, non-mesometric edge,
taking care to keep the point of the scissors within
the uterus close against its wall.
1. The oval thin-walled vesicles lying at intervals
on the walls of the uterus.
2. The presence of the pyriform embryonic area, at
the posterior end of which is seen the primitive
F. & B. 30
466 PRACTICAL DIRECTIONS. [APP.
3. The commencement of the area vasculosa around
the hind end of the area. This is seen better
after treatment with picric acid.
B. Treatment and Examination of the embryo.
a. Preserve the vesicle in picric see I. B. 1.
Stain in haematoxylin, cut out the embryonic
area, leaving a considerable margin, imbed and
cut into sections.
b. In transverse sections observe :
1. At the anterior end of the area the single row of
columnar epiblast and the single row of flattened
2. Immediately in front of the primitive streak be-
tween these two layers a few irregularly shaped
3. Through the middle of the primitive streak,
a. Several ] ay ers of rounded mesoblast cells attached
to, and continuous with, the epiblast in the
middle line, and stretching out laterally beyond
the edge of the area.
b. A single layer of flattened hypoblast.
4. The epiblast outside the embryonic area in the
form of flattened cells and, except in the region
around the primitive streak, overlying a layer of
XV. Examination of an eight days 7 embryo.
A. To obtain the embryo.
The uterus will be found here and there to be
swollen. In these swellings the embryos lie; and
APP.] EIGHT DAYS' EMBRYO. 467
owing to the fact that the wall of the embryonic
vesicle is exceedingly thin, and attached to the ute-
rine wall, they are very difficult to obtain whole.
Cut the uterus transversely on each side of the
swellings and pin the pieces so obtained slightly
stretched out in small dissecting dishes. Cover the
tissue with picric acid solution and allow it to remain
untouched for an hour. Then with two pairs of fine
pointed forceps carefully tear the uterus longitu-
dinally, slightly to one side of the median line of the
free side. This operation will necessarily take some
time, for but a small portion should be done at once,
the picric acid being allowed time to penetrate into
that part of the uterus which has been most recently
With care, however, the student will be able to
open completely the swelling and will observe within
the thin walled vesicle. Great care must also be
exercised in freeing the vesicle from the uterus.
This dissection should be performed with the aid
of a dissecting lens. In case the embryonic vesicle
is burst it will still be possible to extract the embryonic
area which lies on the mesometric side of the uterus ;
the area itself is not attached to the uterine walls.
B. Examination of surf ace view.
1. The increased size of the embryonic area.
2. In the anterior region the medullary folds; di-
verging behind and enclosing between them,
3. The primitive streak.
4. The area opaca now completely surrounding the
468 PRACTICAL DIRECTIONS. [APP.
C. Examination of sections.
Prepare and cut into transverse sections as advised
in XIY. B.
1. In the sections of the anterior region,
(L The lateral epiblast composed of several layers
of columnar cells.
b. The epiblast in the median line one layer thick
and in the form of a groove (medullary groove).
c. The lateral plates of mesoblast.
d. The flattened lateral hypoblast, and columnar
hypoblast underlying medullary groove (noto-
2. In sections through the anterior end of the primi-
Note the continuation of the epiblast, mesoblast
and hypoblast in the middle line.
3. In sections through the posterior end of the area
the same points to be seen as in XIV. B. b. 3.
XVI. Examination of an embryo about 8 days 12 hours.
A. Manipulation as in XV. A.
B. In surface view observe (cf. Fig. 106) :
1. Area pellucida surrounding embryo, outside which
is the well marked area vasculosa.
2. Widely open neural canal, at anterior end dilated,
and partially divided into the three primary vesi-
cles of the brain : note the optic vesicles. At the
posterior end, the sinus rhomboidalis.
3. The mesoblastic somites, 4 to 8.
APP.] FCETAL MEMBRANES. 469
4. The two lateral tubes of the heart, and the com-
mencement of the two vitelline veins.
5. The rudiment of the primitive streak.
6. The commencing head and tail folds.
7. The commencing folds of the amnion.
Compare Fig. 106.
XVII. Examination of the foetal membranes of an embryo
of 14 days.
A. To obtain t/ic embryo, with its membranes.
Manipulate as in XV. A. only dissect under salt
solution instead of picric acid.
B. Observe before removing tJie embryo from the
1. The attachment of the vesicle to the mesometric
side of the uterus over a discoidal area, the
2. The position and form of the placenta.
C. Remove the embryo with its membranes intact,
and observe :
1. the vascular yolk sac, extending completely round
the chorion with the exception of a comparatively
small area where
2. the allantois is situated. The vascularity of the
allantois. The foetal villi projecting into the
maternal placental tissue.
470 PKACTICAL DIRECTIONS. [APR
D. Separate the membranes from one another with-
out tearing them,
and notice :
1. The embryo surrounded by the amnion.
2. The allantois; its position dorsal to the embryo; its
attachment to the chorion ; its circulation.
3. The flattened yolk sac, ventral to the embryo ; its
long stalk; its circulation.
4. The heart.
E. The embryo in surface view.
The points to be observed are
1. The cranial and body flexure, the spiral curvature
of the hinder portion of the body.
2. The vesicles of the brain : cerebral hemispheres,
fore-brain, mid-brain and hind-brain.
3. The eye, and the ear.
4. The heart.
5. The visceral arches and clefts.
6. The fore and hind limbs, and the tail.
APR] NOTES. 471
Since writing the account of section-cutting on p. 434,
we have obtained more experience as to the practical work-
ing of Messrs. Caldwell and Threlfall's microtome there
mentioned. We find that it cuts more accurately and better
than any other microtome with which we are acquainted,
and can confidently recommend it to investigators and
teachers with large classes. In the Cambridge Laboratory,
it is driven by a small water engine and will cut at a rate
of 500 a minute, without detriment to the sections.
Mr Threlfall, of Caius College, has recently elaborated
a method of mounting sections which in our opinion has
many important advantages over the shellac method. It is
as follows. Make a solution of pure india-rubber in benzine
or chloroform. Spread a thin film of this on a clean glass
slide, and allow it to dry. Arrange the sections on the
film y melt the paraffin ; allow the slide to cool, then
immerse the slide for a moment in benzoline (liquid
paraffin), which dissolves the paraffin, and mount in balsam.
The chief advantages of this method are that the sections
do not adhere to the india-rubber until warmed, and they can
be stained after they are fixed on the slide if necessary.
For the latter purpose, wash the benzoline away with
absolute alcohol ; treat with weaker alcohol ; stain ; return
to absolute ; clear with oil of cloves or kreasote, and mount
in balsam (vide Zoologischer Anzeiger, 1883).
Abdominal wall of chick, 281
Albumen : composition of, 3 ;
arrangement of, in hen's egg,
3 ; formation of, in hen, 16 ;
fate of, in hen's egg, 109; of
incubated egg, 185
Alimentary canal of chick, 28 33,
39; of third day and append-
ages of, 171 185 ; mammalia,
Alisphenoid region of chick, 240,
Allantoic arteries : of chick, 225,
293, 298; in mammals, 348,
Allantoic veins of chick, 228, 287,
290; of mammals, 342
Allantoic stalk, 351
Allantois : of chick, 28 33, 46
47, 107, 182 185, 277, 280;
as a means of respiration, 2 32 ;
pulsation of, 277; of rabbit, for-
mation of, 331, 353; of human
embryo, 336340, 355 358 ;
of mammalia, structure of, 348;
of marsupials, 352 : of dog, 358
Alum carmine, to make and use,
Amnion : of chick, 28 33, 43 46,
63, 107, 195; of third day, 113,
276 280; pulsation of, 277,
278; false, of chick, 46; of
rabbit, 330, 353; of human
embryo, 338 340; of mam-
malia, 343 ; structure of mam-
malian, 346; of dog, 358
Amphioxus, spinal cord of, 254
Annuli fibrosi of birds, 210
Anterior commissure of cerebral
hemisphere, mammalia, 381
Aorta of chick, 224, 292, 298;
of mammals, 407
Aortas of chick of second day,
Aortic arches of chick, 103, 106,
167; of fourth day, 225, 291
Apes' placenta, 355 ; histology of,
363 ; derivation of, 364
Aqueductus vestibuli of chick,
Aqueductus sylvii (see iter.)
Aqueous humour: of chick, 153
154; of mammalia, 390
Arbor vitae of birds, 369
Area opaca of chick, 7, 49, 195 ;
mesoblast of, 65 ; hypoblast of,
65 ; vascular portion of, 74 75,
no; of third day, 109
Area pellucida : of chick, 8, 49, 55 ;
of third day, 1 10 ; of mammals,
Area vasculosa : of mammalia,
formation of, 342 ; circulation
of > 343346
Arteria centralis retinas of mam-
malia, 387 390
Arterial system: of chick, 224
226, 291 303; mammalia, 407
Arterial arches, mammalia, 407
Articulare of chick, 244
Attachment of ovum in uterus,
Auditory capsule of chick, 241
Auditory pits of chick, 81, 101
Auricles of chick, 84, 102, 229,
Auricular : appendages of chick of
second day, 102 ; septum of
Avian characteristics, 275
Azygos vein, mammalia. 412
Basi-hyal chick, 245
Basilar: plate, 235 238; mem-
brane, mammalia, 397
Basi-occipital region of chick, 237
Basi-sphenoid of chick, 240, 246
Basi-temporal bone, chick, 246
Beak of chick, 249; formation of,
Biliary ducts of chick, 180 181
Birds, oviparous, 308
Bladder : derivation of, in mam-
mals, 351 ; mammalian, 417
Blastoderm of chick, 4 ; struc-
ture of, in unincubated hen's
e gg> 7 10 ; area pellucida of,
8; formative cells of, 23, 24;
extension of, 26, 27; lateral
folds of, 37 ; head fold of, 27,
37; tail told of, 29, 37; vas-
cular area of, 27 ; hypoblast
f 51; germinal wall of, 52;
epiblast, 55 ; of third day, 109,
Blastoderm of mammal, forma-
tion of layers of, 314325 ; vas-
cular area of, 326 ; pellucid
area of, 328; head and tail
Blastodermic vesicle, 314 316,
319 ; outer layer of, 314; inner
mass of, 314 ; to examine, 465
Blastopore of mammalian ovum
(van Beneden's), 314; of chick
and mammals, see neurenteric
Blood islands of vascular area of
Blood corpuscles of chick, for-
mation of, 92 94
Blood-vessels : of area opaca of
chick, formation of, 92 94 ;
development of, practical di-
rections for study of, 459, 460
Body cavity : of chick, 39 ; forma-
tion of, 40, 41 ; posterior medi-
astinum of, 267 ; of mammalia,
Body flexure of chick, 196; on
third day, 116
Body flexure : in rabbit, 334 ; in
dog, 334 ; of human embryo,
Borax carmine, to make and use,
Brain: of chick, 117 123, 281 ;
of mammalia, 367 387 ; divi-
sions of, 367 ; hind brain, 367
370; mid brain, 370, 371; fore
brain, 371385 ; histogeny of,
Branchial clefts and arches (see.
Breeding mammals for study, 460
Bronchi, mammalian, 418
Bronchial tubes of chick, 177
Bulbus arteriosus of chick, 84, 225,
229, 257; septum of, 257, 259,
260 262 ; of mammalia, 407
Caecum, mammalia, 419
Canales Botalli (see Ductus Bo-
Canalis auricularis of chick, 257,
Canalis reuniens, 160; auricularis
of chick, 169, 229; reuniens of
ear of mammalia, 393 398
Cardinal veins : of chick, 1 70 ; 284
285 ; anterior and posterior
of mammalia, 409 4 1 3
Carnivora, placenta of, 358
Carotid: common artery of chick,
295, 298; external and internal
artery, 292, 295 ; of bird and
Carpus of chick, 234
Cartilage bones, 242 ; of skull of
Cerato-hyals of chick, 245
Cerebellum: of chick, 122, 203,
368 370 ; of mammalia, 367
370; ventricle of, 368; cho-
roid plexus of, 368; pyramids,
and olivary bodies of, 368 ;
arbor vitae, flocculi of, 369 ;
pons varolii of, 369, 370; velum
medullas ant. 370
Cerebral hemispheres : of chick,
117; of mammalia, 376 385;
ventricles of, 377; lamina ter-
minalis, 377; corpus striatum,
378; commissures of, 381 383;
septum lucidum, 383 ; fissures
Cerebral vesicles of chick, 200 ;
of second day, 79, 100
Cerebro-spinal canal in chick, 40
Cerebrum of mammalia, mono-
tremata, iusectivora, 384
Cheiroptera, placenta of, 353
Chest wall, of chick, 281
Chorion : of hen's egg, 47 ; of
mammal, true and false, 348;
of rabbit, true and false, 353 ;
of human ovum, 355358;
of dog, 358
Chorion leve, 356 358
Chorion frondosum, 356 358
Chorionic villi of mammal, 340
Choroid coat of eye, of chick,
Choroid plexuses of mammalia,
Choroidal fissure of chick, 136
141, 147 149; of mammalia,
Chromic acid, 427 428
Ciliary : ganglion of chick, 128 ;
ridges of chick, 142 ; muscles,
Circulation : in chick of second
day, 105; of third day, no
113; of chick, later stages,
Circulatory system of chick, re-
Clavicle : man, 405 ; of chick, 234
Clinoid ridge, posterior, chick,
Clitoris, mammalia, 417
Cloaca of chick, j 74 ; mammalia,
Cochlea of chick, 203
Cochlear canal, mammalia, 390
Cock, coni-vasculosi, parepidi-
dymis and vas deferens of, 224
Columella of chick, 166, 245
Commissures of spinal cord, 253,
Coni-vasculosi of cock, 224
Cornea of chick, 150 153; of
Cornu ammonis, (see Hippoc.
Coracoid of chick, 234
Coronary vein, mammalia, 409
Corpora bigemina of chick, 121
Corpora mammilaria, 378
Corpora quadrigemina of mam-
malia, 370; geniculata, 371
Corpus albicans, 373
Corpus callosum : mammalia, 381 ;
rostrum of, 383 ; of marsupials,
383 ; of monotremes, 383
Corpus luteum, 311
Corpus striatum, mammalia, 378
Corrosive sublimate, how to use,
Cotyledonary placenta, derivation
Cranial flexure : of chick, 1 16, 196 ;
of second day, 101 ; of rabbit,
333; of human embryo, 338
Cranial nerves : of chick, 123 129,
203 ; of second day, 101 ; de-
velopment of, 127 129; of
Cranium of chick, 235 242 ;
cartilaginous, 242 ; cartilage
bones of, 242 ; membrane bones
Cranium, mammalia* 401
Crura cerebri, 371
Crypts of placenta, 360 363
Cumulus proligerus, 310 '
Cupola, 397, 398
Decid.ua : of human placenta, 356 ;
reflexa in human, 356 358 ;
vera, 356 358; serotina, 356
358; reflexa in dog, 359^
Deciduate placenta, 352 ; histology
Dentary bones, 246
Dentine, mammalia, 421
DESCEMET'S membrane, chick, 151
Diaphragm, muscles of, 211;
Diffuse placenta, 359 ; histology
Discoidal placenta, 353
Dog, placenta of, relation with
placenta of rabbit, 358
Dorsal aorta of chick, 167
Ductus arteriosus, man, 408
Ductus cochlearis of chick, 159
Ductus Botalli of chick, 287, 289,
296; of mammalia, 408
Ductus Cuvieri of chick, 170, 228,
Ductus venosus of chick, 169, 226 ;
of mammalia, 413
Duodenum of chick, 172 174
Ear: of chick, 156 161 ; of mam-
malia, 390 397 ; accessory
structures of, 397 399
Egg tubes of Pfliiger, 222
Egg membranes of mammal, 310
Egg, to open, 437, 438
Elephas, placenta of, 358
Embryo of chick : directions for
examining, 439459 ; of 36
48 hours, 437 444; of 48 to
50 hours, 444 447 ; of third
day, 447 45 1 ; of fourth day r
451 453; of 20 hours, 453
456; before incubation, 457;
segmentation, 458; blood-ves-
sels of, 459
Embryo of mammals : directions
for examination of, 461 470 ;
of segmenting ova, i 72 hours,
461 464; of blastodermic vesi-
cle of, 72 90 hours, 465 ; of 7
days, 465 ; of 8 days, 466 ; of
8 days 12 hours, 468 ; of 14
days, 469 ; of foetal mem-
Embryonic area of rabbit, 317;
composition of, 317
Embryonic membranes: in mam-
malia, ideal type, 342 352 ;
yolk sac of, 345 351 ; amniori
of, 345 351 ; allantois of, 345
351; zona radiata of, 345; se-
rous membrane of, 345 ; cho-
rion of, 345 ; shedding of, at
birth, 351; monotremata, 352;
marsupialia, 352 ; rodentia,
353, 354 ; insectivora, 353 ;
cherioptera, 353 ; man and
apes, 355 358; carnivora, 358 ;
hyrax, 358; elephas, 358 ; oryc-
teropus, 358, horse, '359 ; pig,
359 ; lemurs, 359
Embryonic sac in chick, 37 38
Embryonic shield of chick, 49,
Endolymph, mammalia, 396
Epiblast : formation of, in chick,
25, 26; derivation of, 26; of
rabbit embryo, 316 ; histological
differentiation of, in chick, 271;
epidermis, 271; nervous system,
271 ; sense organs, 272 ; mouth,
272 ; anus, 272; pituitary body,
272; salivary glands, 273; of
blastoderm from 8th to* 1 2th
Epididymis, mammalia, 415
Epiotic of chick, 246
Epithelioid lining of heart of
Epithelium of throat of chick, 182
Epoophoron, of hen, 224
Ethmoid : region, chick, 240 ;
lateral, 241 ; bone, chick, 246
Eustachian tube: of chick, 165;
of rabbit, 334; of mammalia,
Eustachian valve : of heart of
chick, 263 4
External auditory meatu* of mam-
External carotid artery, chick, 225
Eye; of chick, 200; development
of, 132 155 ; of mammalia,
Eyelids, of chick, 155; of mam-
Face of chick, 246; of human
Facial nerve (see Seventh)
Falciform ligament, mammalia,
Fallopian tubes, mammalia, 415
False amnion of chick, 46
Falx cerebri mammalia, 377
Fasciculi teretes, 368
Feathers, formation of, 282
Female pronucleus, 17
Femur, chick, 234
Fenestra ovalis, of chick, 166, 245 ;
Fenestra rotunda of chick, 166,
245 ; mammalia, 398
Fibula, chick, 234
Fifth nerve of chick, 126 129,
Fifth ventricle of man, 383
First cerebral vesicle of chick,
second day, 97
Fissures of spinal cord, 254
Flocculi of cerebellum of birds, 369
Foetal appendages : of chick, 276
280; amnion, 276 278; allan-
tois, 277; yolk-sac, 277; mem-
branes of mammal, to examine,
Foldmg-off of embryo chick, 113,
Follicle, ovarian, 12 15
Foramen ovale : of heart of chick,
262, 264, 289, 297, 302
Foramen of MONRO, 372
Fore brain : of chick, 100 ; of rab-
bit, 329; of mammalia, 371
385; optic vesicles of, 387 390;
thalamencephalon, 371 376 ;
cerebral hemispheres, 376
385 ; olfactory lobes, 385
Foregut of chick, formation of,
Formation of the layers in mam-
mals, 314 325
Formative cells, 23 24
Fornix, mammalia, 381 ; pillars
Fourth ventricle, chick, 122 ;
Fourth nerve, chick, 128
Fretum Halleri, chick, 229
Frontal bones, chick, 246
Fronto nasal process, chick, 165,
Gall-bladder of chick, 181
Gasserian ganglion, chick, 128
Generativeglands : of chick, 220
224; of mammalia, 414 415
Generative organs, external, mam-
malia, 415 417
Genital cord, mammalia, 415 '
Genital ridge, chick, 220
Germ cells, primitive, of chick,
Germinal disc of chick, 12
Germinal epithelium, 213
Germinal layers of chick, 26
Germinal vesicle of chick, 1 2
Germinal wall, 52 ; structure of,
65 66; function of, 66
Glomeruli of kidney of chick,
Glands, epidermic, of mammalia,
Glomerulus of Wolffian body of
Glossopharyngeal nerve (see Ninth
Gold chloride, 460
Graafian follicle, chick, 222, 310
Grey matter, of spinal cord of
chick, 253; of brain of mam-
Growth of embryo of chick, 70
Guinea-pig, structure of blasto-
derm of, 323; relation of em-
bryonic layers of, 323; inver-
sion of the layers in, 341
Haematoxylin, to make and use,
Hardening reagents, 425 428;
picric acid, 425 ; corrosive sub-
limate, 426 ; osrnic acid ; 427 ;
chromic acid, 427 ; absolute
alcohol, 428 ; the necessity of,
Head of chick, 200 ; of rabbit,
Headfold of chick, 2729, 33 37;
16 to 20 hours, 60 ; 20 to 24
hours, 66 ; of second day, 77 ;
of mammal, 329
Heart of chick, 229 230, 256
264; formation of, 82 89, 102 ;
beating of, on second day, 89 ;
of third day, 167; auricles,
2 p 262 ; ventricles, 260 262 ;
auricular septum, 257 262;
ventricular septum, 2 57 ; canalis
reuniens, 257 259; bulbus ar-
teriosus, 257 262 ; foramen
ovale, 262 264 ; Eustachian
valve, 263 264; circulation in,
263 264; structure of, 287
289, 293' 297 ; resume of, 299
Heart of mammals, 329; struc-
ture of, 331 ; formation of, 406 ;
comparison of, with birds, 407
Hemiazygos vein, mammalia, 412
Hen: formation of albumen in,
1 6 ; ovarian follicle of, 12 15 ;
mesovarium of, 1 1 ; ovary of,
1 1 ; ovarian ovum of, 1 1 , 15;
oviduct of, 15; epoophoron,
paroophoron and oviduct, 224
Hen's egg, albumen of, 3, 16;
blastoderm, 7 10, 26, 27 :
chalazae, 4 ; cicatricula, 4 ; im-
pregnation of, 17 ; laying of,
17; polar bodies of, 17; seg-
mentation of, 1 8 24; vitelline
membrane of, 4, 13 15 ; yolk
of, 4 7 ; chorion of, 47 ; shell
of, i, 16; irregular develop-
ment of, 48, 49 ; segmentation,
cavity of, 50
Hepatic cylinders of chick, 1 79 ;
circulation of chick, 227 ; veins,
Hind brain: of chick, 100 ; of
rabbit, 329 ; of mammals, and
birds, 367 370 ; medulla of,
367 ; cerebellum of, 367 370
Hippo-campus major, mammalia,
Hippo-campal fissure of cerebrum
of mammalia, 385
Histological differentiation, in
chick, 269 273 ; of epiblast,
269, 271; of hypoblast, 269;
of mesoblast, 269
Histology of placenta, 359
Holoblastic segmentation, 307
Human embryo: villi of, 335;
early stages of, 335 ; allantois
of, 336 340; yolk-sac of, 336
340 ; medullary plate of, 337 ;
amnion of, 338 340; cranial
flexure of, 338 340; limbs of,
339; body flexure of, 339
340; face of, 340; relation of,
with other mammals, 341 ; pla-
centa of, 355
Human ovum, size of, 307
Human placenta, histology of,
363 ; derivation of, 364
Humerus, chick, 234
Hyaloid membrane, chick, 144,
Hyoid arch of chick, 243 245 ;
of rabbit, 334; of mammalia,
Hyoid bone of chick, 245
Hypoblast of chick : formation of,
2 5> 5 1 ' 59 > derivation of, 26;
of area opaca, 65 ; histological
differentiation of, 269; of di-
gestive canal, 272 ; of respira-
tory ducts, 272 ; of allantois,
273; notochordal, 273
Hypoblast of rabbit embryo, 316,
Hypoblastic mesoblast of chick,
59 62; of mammal, 321
Hypogastric veins : chick, 289 ;
mammalia, 411 413
Hypohyal, mammalia, 403
Hypophysis cerebri (see Pituitary
Hyrax, placenta of, 358
Ischium, chick, 234
Island of Eeil, 385
Iter a tertio ad quartum ventricu-
lum, 121, 370
Jugal bones, chick, 246
Jugular vein, 284 290
Kidney: of chick, 218220; tu-
bules of, 219 ; of mammalia,
Ileum, chick, 234
Iliac veins, mammalia, 411 413
Imbedding, methods of, 432 434
Impregnation of hen's egg, 17;
of ovum of mammal, 310 312
Incubators, makers of, and how
to manage, 423
Incus, mammalia, 398, 404
Inferior cardinal veins, chick, 228
Infundibulum : chick, 119 121;
ventricle of, 373 ; tuber cinereum
of, 373 ; of mammalia, 372 ; of
Inner mass of segmented ovum,
314 ; of blastodermic vesicle,
Innominate artery of chick, 296
Insectivora, placenta of, 353
Intercostal veins, mammalia,
Interhyal ligament, 403
Intermediate cell mass of chick,
95, 189, 190
Internal carotid artery, chick, 225
Inter-nasal plate, chick, 240
Inter-orbital plate of chick, 240
Intervertebral ligaments, mam-
Intervertebral regions, chick, 207,
Intestine, mammalia, 419
Inversion of the layers, 341
Labia majora, mammalia, 416
Lacrymal bones, chick, 246 ; ducts,
chick, 155, 156; glands, chick,
r 55> 156; groove, chick, -248;
duct, mammalia, 390
Lagena, chick, 1^9; birds, 397 ,
Lamina, dorsalis of chick, 29, 62
Lamina spiralis, mammalia, 397
Lamina terminalis, mammalia,
Large intestine of chick, 174
Larynx of chick, 177
Lateral folds of blastoderm of
chick, 37 ; of chick of second
Lateral plates of mesoblast, 68
Lateral ventricles of chick, 117 :
of mammalia, 377 ; cornua of,
Laying of eggs, 17
Legs of chick, 200
Lens, chick, formation of, 134,
Ligamenta suspensoria, of birds,
Ligamentum, pectinaturn, 144 ;
vesicae medium, 351
Ligamentum longitudinale an-
terius and posterius, mammalia,
Limbs, of chick, 198 200, 233 ;
of rabbit, 334 ; of human em-
bryo, 339 ; mammalia, 406
Liver of chick, 178 181 ; mam-
Lumbar veins, mammalia, 412
Lungs of chick, 176 178, 267 ;
Male pronucleus, 17
Malleus, 398, 404
Malpighian corpuscles, chick, 182 ;
bodies of chick, 190
Mammalia, two periods of develop-
ment, 308 ; viviparous, 308
Mammary glands, 366; a source
of nutriment for the embryo,
Man (see Human embryo)
Mandible, chick, 246
Mandibular arch, chick, 242
244; maxillary process of,
chick, 243; rabbit, 334; mam-
Manubrium of malleus, 403
Marsupialia, foetal membranes of,
Maturation of ovum of mammal,
Maxilla bones, chick, 246
Maxilla-palatine bones, chick, 246
Maxillary, processes of mandibu-
lar arch of chick, 243
Meatus auditorius externus, of
chick, 1 66; of mammal, 397
Meatus venosus, of chick, 169,
Meckelean cartilage, chick, 244;
Medulla oblongata, of chick, 122 ;
of mammalia, 367
Medullary canal, of chick, 40, 62,
Medullary folds, of chick, 40, 62,
66, 77, 97 ; of mammal, 327
Medullary groove, of chick, 29,
62 65 ; of rabbit, 320, 32 1 ;
of man, 338 ; closure of, in
mammal, 327 331
Medullary plate, of chick, 62 ; of
rabbit, 320 ; of man, 338
Membrana capsulo pupillaris of
mammalia, 387 389
Membrana limitans externa, 145;
Membrana propria of follicles,
Membrane : of shell of hen's egg,
T ; serous, of chick, 32 41 ;
vitelline of hen's egg, 13 15
Membrane bones, 242 ; of skull,
Membrane of Keissner, mamma-
Membrane of Descemet, 389
Membrane of Corti, and tectoria
Membranous labyrinth, chick,
Meniscus of birds, 210
Meroblastic segmentation, 18
Mesenteric veins of chick, 228,
Mesentery, of chick, 173; mam-
malia, 419 20
Mesoblast: derivatives of, in chick,
25 26; of primitive streak of
chick, 54, 57; derived from
lower layer cells in chick, 55,
57, 59 ; of area opaca in chick,
65 ; splitting of, in chick, 68 ; of
trunk of embryo chick, 185
189 ; histological differentiation
of, in chick, 269; of primitive
streak of rabbit, 320; of mam-
mal, double origin of, 321
323; vertebral zone of, 328;
lateral zone of, 328 ; somites
Mesoblastic somites, formation of
in chick, 70; of chick, 81, 185
187, 204 208
Mesocardium of chick, 88 ; forma-
tion of, 264
Mesogastrium, chick, 182
Mesonephros of chick, 212
Mesovarium of fowl, 1 1
Metacarpus, chick, 234
Metadiscoidal placenta, histology
of, 362 ; derivation of, 364
Metamorphosis of arterial arches,
bird and mammalia, 408
Metanephos (see Kidney)
Metanephric blastema, of chick,
Microtomes, and makers of, 434
435; 47 1
Mid brain: of chick, 100, 200; of
rabbit, 329 ; of mammalia, 370
371 ; ventricle of, 370; nates
and testes of, 371; corpora
geniculata, and crura cerebri of,
Monotremata, foetal membranes
Mouse, inversion of the layers in,
Mouth, chick, 249, 281 ; of rabbit,
formation of, 334
Miillerian duct : chick, 214 2 1 8 ;
mammalia, 414 415
Muscle plates of chick, 187 189,
204 208, 21 1 ; segmentation
Muscles: hyposkeletal, chick, 211 ;
episkeletal, chick, 211; cuta-
neous, chick, 2 1 1 ; extrinsic and
intrinsic of limb, chick, 212
Muscular walls of heart of chick,
Nails, of chick, 283
Nares : posterior, chick, 251; an-
terior and posterior, of mam-
Nasal capsule, chick, 242 ; car-
tilages, chick, 246; bones, chick,
246; groove, chick, 246; pro-
cesses of chick, inner, 248;
outer, 248; labyrinth, chick,
Nasal organ (see Olfactory organ)
Nasal pits, of birds, 71 ; chick,
Nates of mammalia, 371
Nerves, of chick of second day,
101 ; of mammalia, 400
Nervous system of mammalia,
Neural band, chick, 123; crest,
Neural canal of chick, 31 39, 66;
second and third day, 122 ; de-
velopment of, 251 256
Neurenteric canal, of chick, 71
74, 175; mammalia, 399; of
mole, 326, 328
Ninth nerve, chick, 126 129, 203
Node of Hensen, 319
Non-deciduate placenta, 352
Nose, chick, 249
Nostrils, chick, 251
Notochord: of chick, 29, 60 62,
208 210, 237 238; of second
day, 101; sheath of chick, 208;
of mammal, 323, 400; forma-
tion of, 325
Nuclei, 1 6
Nucleus of Pander, 7
Nucleus pulposus, of birds, 210,
Nutrition of mammalian embryo :
308 ; by means of placenta, 350
Occipital: supra-, basi-, ex-, of
chick, 246 ; foramen, chick, 237
(Esophagus of chick, 173 ; mam-
Olfactory organ of chick, 161 ;
nerve of chick, 162 ; grooves,
chick, 202 ; lobes of mammalia,
Olivary bodies, 368
Omentum, mammalia, lesser, 420;
Opisthotic of chick, 246
Optic vesicles : of chick of second
day, 79, 97 ; chick, 133134 :
formation of, 141 144 ; of
Optic lobes, chick, 121
Optic nerves, chick, 133, 146
Optic cup, 134
Optic chiasma, chick, 147; mam-
Optic thalami of mammalia, 373
Orbitosphenoidal region, chick,
Organ of Corti, mammalia, 395
Organ of Jacobson, mammalia,
Orycteropus, placenta of, 358
Osmic acid, how to use, 427
Osseous labyrinth, chick, 158
Otic vesicle, chick, 157
Outer layer, of blastodermic vesi-
Ova, primordial, of chick, 221
Ovarian follicle : of hen, 12 15 ;
Ovarian ovum: of hen, n 15;
of mammals, 309
Ovary: of adult hen, n ; of
chick, 222; of mammals,
309 ; follicles of, 309 ; corpus
luteum of, 311.
Oviduct of adult hen, 15 ; of
Oviparous animals, 308
Ovum : of birds and mammals
compared, 307 ; of mammal
in follicle, 309 ; membranes of,
310; maturation and impreg-
nation of, 310 312; polar
bodies of, 311 ; segmentation
of, 312 314; blastopore of
Palate, mammalia, 420, 421
Palatine bones, chick, 246
Pancreas: of chick, 181 ; mam-
Pander, nucleus of, 7.
Parachordals, chick, 235 238
Parepididymis of cock, 224
Parietal bones of chick, 246
Parieto-occipital fissure of cere-
brum of man and apes, 385
PARKER on the fowl's skull, 245
Paroophoron of hen, 224
Pecten, chick, 147
Pectoral girdle, chick, 234; mam-
Pelvic girdle, chick, 234 ; mam-
Penis, mammalia, 417
Pericardial cavity, chick, develop-
ment of, 264 269 ; of rabbit,
331; mammalia, 406
Perilymph, mammalia, 396
Periotic capsules, chick, 237
Peritoneal covering of heart of
chick, 88 ; cavity, mammalia,
Peritoneum, mammalia, 4 19 420
PFLUGER, egg tubes, 222
Phalanges, chick, 234
Pharynx, mammalia, 418
Picric acid, how to use, 425
Picro-carmine, to make and use,
Pig, placenta, histology of, 360
Pineal glands, chick, 117 119;
of mammalia and birds, 373
Pituitary body : chick, 119 1 2 1 ;
rabbit, 334; of birds, 372;
mammalia, 372, 420
Pituitary space, chick, 240
Placenta : 342 ; discoidal, deci-
duate, type of, 353, 354; meta-
discoidal, type" of, 354358 ;
decidua of, 356; chorion laeve
of, 356 358; chorion frondo-
sum of, 356 358 ; comparison
of, 358; zonary type of, 358;
diffuse form, 359 ; polycotyle-
donary form, 359 ; histology of,
3593^3; evolution of, 364;
of sloth, 360.
Pleural cavity, chick, development
of, 264 269 ; mammalia, 406
Pleuroperitoneal space of chick,
28 33, 84; formation of, 40,
Pneumogastric nerve (see Tenth
Polar bodies, 1750! ova of mam-
mals, 3 r i
Polycotyledonary placenta, 359 ;
histology of, 360
Pons Varolii of birds, 369 ; of
Position of embryo chick of third
and fourth days, 113 116
Postanal gut, of chick, 175; of
rabbit, relation of, to primitive
Posterior nares, chick, 202
Potassium bichromate, 460
Premaxilla bones, chick, 246
Prenasal bones of chick, 246
Presphenoid region, chick, 240
T, ? 46
Primitive groove of chick, 56 ; of
Primitive streak of chick, 52 62 ;
of chick from 20 to 24 hours,
70; of rabbit, 319
Processus infundibuli, chick, 121
Proctodasum of chick, 175; of
Pronucleus, female, 17; male, 17
Prootic, chick, 246
Protovertebrae (see Mesoblastic
Pterygo-palatine bar, chick, 243
Pterygoid bones, chick, 246
Pubis, chick, 234
Pulmonary veins of chick, 228,
Pulmonary arteries of chick, 294
298; mammalia, 407
Pupil, chick, 142
Pyramids of cerebellum, 368
Quadrato-jugal bones, 246
Quadrate, chick, 243
Kabbit embryo, growth of, 327
334; placenta of, 353
Badius, chick, 234
Eat, inversion of the layers in,
Eecessus labyrinthi, mammalia,
39 39 8
Eecessus vestibuli (see Aqueductus
vestibuli) chick, 203
Eespiration of chick, 303 ; of third
Eete vasculosum, mammalia, 4 14
Eetina, chick, 142, 144146
Eibs, chick, 234; mammalia, 405
Eodentia, placenta of, 353
Eods and cones of retina, chick,
Eostrum, chick, 246
Euminants' placenta, histology of,
Sacculus hemisphericus, mam-
malia, 390 398
Salivary glands, mammalia, 420
Scala media (see Cochlear canal)
Scala tympani, mammalia, 395
Scala vestibuli, mammalia, 395
Scapula of chick, 234
Sclerotic coat of eye of chick, 141
Sclerotic capsules, mammalia, 405
Scrotum, mammalia, 416
Sebaceous glands, 366
Secondary optic vesicle (see Optic
Sections, method of cutting, 434
436 ; mounting of, 436
Segmentation: of hen's egg, 18
24; meroblastic, 18; of mam-
malian ovum, 312 314; of
hen's egg to observe, 458; of
mammalian ovum to observe,
Semicircular canal : of chick,
158 ; mammalia, 390 398
Semi-lunar valves, chick, 258
Sense capsules of chick, 211 212
Septum lucidum, mammalia, 383
Septum-nasi, chick, 246
Serous membrane of chick, 32
Serous envelope of chick, 107 ;
of mammals, 346
Seventh nerve of chick, 127 129,
Shell-membrane of chick, i
Shell of hen's egg, i ; formation
Shield, embryonic, of chick, 49
Sinus rhomboidalis : of embryo
chick, 71, 81 ; of rabbit, 329
Sinus terminalis, of chick of
second day, 91, 104; in rabbit,
Sinus venosus of chick, 169, 226,
Skeleton of limb, chick, 234
Skull of chick, 235 251 ; cartilage
and membrane bones of, 246 ;
of mammalia, 401 405
Sloth, placenta, histology of, 360
Somatic stalk of chick, 29 42 ;
of mammals, 351
Somatopleure of chick, 29 33;
formation of, 4041, 68
Spermatozoa of chick, 223
Spinal nerves : of chick, 123; de-
velopment of, 129132 ; of
Spinal cord of chick: develop-
ment of, 251 256 ; white mat-
ter of, 252; grey matter of,
253; canal of, 252 256; epi-
thelium of, 251, 252; anterior
grey commissure of, 256 ; an-
terior fissure of, 254 256;
dorsal fissure of, 255 256;
posterior grey commissure of,
256; sinus rhomboidalis of,
256; anterior columns of, 256;
posterior columns of, 256 ;
lateral columns of, 256; an-
terior white commissure of,
256; posterior white commis-
sure of, 256
Splanchnic stalk of chick, 29
Splanchnopleure of chick, 29
33 ; formation of, 40 42, 68
Spleen of chick, 182
Splint bones of chick, 246
Squamosal bones of chick, 246
Staining reagents, 428 432; has-
matoxylin, 429 ; borax carmine,
430; carmine, 431; picro-car-
mine, 431 ; alum carmine, 431
Stapes, of chick, 245 ; mammalia,
Sternum of chick, 235 ; of mam-
Stomach of chick, r 73 ; mam-
Stomodaeum, of chick, 119, 203;
Stria vascularis, mammalia, 397
Subclavian arteries of chick, 296
298. of mammalia, 409
Subclavian veins, mammalia, 409
Sulcus of Monro, 373
Superior maxilla of chick, 165 ;
maxillary processes of chick,
202; of rabbit, 334
Superior cardinal veins of chick,
Supra-renal bodies, mammalia,
structure of, 413; relation of,
with sympathetic nervous sys-
Subzonal membrane of mammal,
Sylvian fissure, mammalia, 384,
Sympathetic nervous system of
Tail-fold of chick, 29 37, 196;
of second day, 96 ; of mammal,
3 2 9
Tail-swelling of chick, 74
Tarsus of chick, 234
Teeth, mammalia, 421
Tela choroidea, 375
Tenth nerve of chick, 125, 127
129, 203 ^
Testis of chick, 222, 371
Thalamencephalon : of chick,
117; of mammalia, 371 376;
ventricle of, 372; floor of, 372,
373; sides of, 373; roof of, 374
Third nerve of chick, 129
Third ventricle of mammalia, 372
Throat of rabbit, formation of,
Thyroid body, of chick, 181 ;
Tibia of chick, 234
Tongue of chick, 282
Trabeculae of chick, 236, 239 241
Trachea of chick, 176, 177 ; mam-
Tuber cinereurn, 373
Turbinal bones of chick, 246
Tympanic cavity of chick, 166 ;
membrane of chick, 166 ; cavity
of mammalia, 397, 418; mem-
brane of mammalia, 397
Ulna, of chick, 234
Umbilical, arteries (nee Allantoic);
veins (see Allantoic veins); vesi-
cle of mammals (see Yolk-sac) ;
stalk of chick of third day, 113;
Ureter of chick, 219; mammalia,
Urethra, mammalia, 417
Urinogenital organs of mam-
malia, 414 417; sinus of mam-
Uterine crypts, 350
Uterus, mammalia, 415
Utriculus of mammalia, 393 398
Uvea of iris, chick, 144
Valve of Vieussens, of birds, 369 ;
of mammals, 370
Vagina mammalia, 415
Vagus nerve (see Tenth nerve)
Vasa efferentia and recta mam-
Vascular system of chick, 224
230; of second day, 89 94, 102
106 ; of third day, 167 170 ;
mammalia, 406 413
Vascular area: of blastoderm of
chick, 27; of third day, no
113; of rabbit's ovum, forma-
tion of, 326
Vas deferens : of cock, 224 ; mam-
Velum medullas anterius (see
Valve of Vieussens) ; posterius,
Vermiform appendix, mammalia,
Vena cava, inferior, of chick, 228,
285 290 ; mammalia, 409
Venae cavae, superior, of chick,
286 290 ; of mammalia, 409
Venae advehentes of chick, 227,
287 289 ; revehentes of chick,
227, 287 289
Vena terminalis (see Sinus termi-
Venous system: of chick, 226
229, 283 290,301 303; mam-
malia, 409 413
Ventricles of brain of chick of
second day, 102; of mammals,
117, 121 122; of chick, 229
Ventricular septum, chick, 230,
Vertebrae of chick, primary, 205
208 ; permanent, 205 208 ;
bodies of, 207 209
Vertebral arches, osseous, of
chick, 207, 210; mammalia,
Vertebral artery of chick, 295
Vertebral column, of chick, 205
208 ; membranous, 205 208 ;
secondary segmentation of, 205
208 ; explanation of do., 205
206 ; of mammalia, early de-
velopment, ossification of, 400,
Vertebrate animal, general struc-
ture of, 39
Vesicle of third ventricle (see
Vessels of placenta, 360 363
Vestibule, chick, 158
Villi : of human ovum, 335 ; of
zona in dog, 347; of subzonal
membrane of rabbit, 347 ; of
chorion of mammal, 349; of
Visceral arches, 245 ; of rabbit,
Visceral arches of chick, 162 167;
of rabbit, 334; of mammalia,
Visceral clefts: of chick, 162
167, 281; closure of do., 164;
of rabbit, 334; of mammalia,
Visceral folds of chick, 163
Visceral skeleton of chick, 242
Visceral vein of chick, 284 290 ;
of mammalia, 409 413
.Vitelline arteries: of chick, 167,
293 298, 225; of second day,
Vitelline duct of chick, 196, 232 ;
of mammals, 350
Vitelline membrane, 4; of hen's
egg, 1 3 1 5 ; of mammal, 310
Vitelline veins of chick, 84, 226,
288 290 ; of second day, 92,
104; in rabbit, 343; of mam-
malia, 410 413
Vitreous humour of chick, 140, 150
Viviparous animals, 308
Vomer of chick, 246
White matter : of spinal cord of
chick, 252; of brain of mam-
malia, 386 387
Wings of chick, 200
Wolffian body: of chick, 190
193; of mammalia, 4/4; of
chick of second day, 106
Wolffian duct of chick, 190, 213 ;
of second day, 94 95, 106; of
Wolffian ridge of chick, 198
Wolffian tubules of chicK, 106,
Yolk of hen's egg, 4 7 ; arrange-
ment of, 6; structure of, 5
Yolk-sac: of chick, 28 37, 277
280; of mammals, 327; of
marsupials, 352; of rabbit, 353;
of human ovum, 355 358; of
Zona radiata, 310; of chick, 15
Zonary placenta: histology of,
360 ; derivation of, 364
CAMBRIDGE : PRINTED BY C. J. CLAY, M.A. & SON. AT THE UNIVERSITY PRESS.
UNIVERSITY OF CALIFORNIA BERKELEY
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